But experts disagree about whether pit bulls are inherently more dangerous -- or just the latest breed in vogue among irresponsible dog owners.

After all, German shepherds killed more people than any other dog in the late 1970s, when many people favored the breed for its fierce reputation. Then, for two years, it was Great Danes. Rottweilers topped the list of killer dogs through most of the '90s, according to a study by the Centers for Disease Control. Now it's pit bulls.

And even those rankings are based only on the rarest of dog attacks -- the couple dozen each year that kill.

Much less is known about which breeds are most likely to cause nonfatal bites, which send an estimated 1,000 people to hospital emergency rooms each day around the country. There's no central reporting agency that tracks the estimated 4.7 million U.S. dog bites each year. And smaller studies present conflicting results for which breeds are the most dangerous.

"If we're just focusing on dog deaths and we're just focusing on pit bulls, we're missing the point," said Florida dog trainer Jim Crosby, a national expert on dog aggression.

The lack of reliable data could make it more difficult to figure out how to draft effective regulations to keep the public safer from dog bite injuries or which breeds to focus on. In the wake of several pit bull attacks, including the one that killed 12-year-old Nicholas Faibish on June 3, San Francisco officials are supporting state legislation to give them the authority to crack down on that particular breed.

"I've got a pit bull problem," said Carl Friedman, director of San Francisco's Animal Care and Control department, which responds to dog bites in the city. Friedman points out that most of its hearings on aggressive dogs involve pit bulls.

State law bars cities and counties from targeting specific breeds. But a bill sponsored by Sen. Jackie Speier, D-Hillsborough, would let cities restrict breeding of certain breeds, or force owners to spay or neuter the dogs to make them less aggressive.

Still, despite the recent wave of publicity surrounding pit bull attacks, the number of fatal dog maulings has remained fairly constant -- averaging around 20 per year in the United States for decades. Indeed, you're more likely to be struck dead by lightning than killed by a dog.

But the breed responsible for the fatalities has changed. Lately, pit bulls have been the main culprit, accounting for 45 of the 145 fatalities since 1999, according to a Chronicle analysis of dog fatality data collected by the National Canine Research Foundation. Rottweilers ranked second with 25 attacks.

Other unexpected breeds have killed people, too. For instance, a tiny Pomeranian mix climbed up on a bed and killed a 6-week-old girl in Southern California in 2000. Because fatal maulings are so rare, some dog experts say it's unfair to blacklist an entire breed based on a few vicious attacks.

"You can't base your assumptions about a whole breed's behavior on three or four dogs," said Karen Delise, founder of the National Canine Research Foundation, who has conducted extensive dog fatality studies.

While even their defenders concede that powerful breeds, like pit bulls and Rottweilers, can kill more easily than miniature poodles or cocker spaniels, many insist the deaths have more to do with fads in ownership than problems with the breed itself.

"In the early to mid-1990s, Rottweilers became the tough-guy dog," said Crosby. "They were the macho dog to own amongst people who were not particularly responsible owners." Now it's pit bulls, says Eric Sakach of the United States Humane Society. Sakach said some people are specifically breeding pit bulls for fighting and aggression, which in turn can lead to more deadly attacks.

Kenneth Phillips, a Southern California lawyer who has devoted his career exclusively to dog bite cases, says all kinds of dogs bite -- not just the ones people think of as dangerous.

Indeed, some of the most severe injuries his clients have faced came from dachshunds. "If they bite you, they just rip off your face," he said.

Another limitation with dog bite statistics is they generally do not take into account the popularity of the dogs: One breed may account for more attacks than another, simply because the breed is more common.

The American Kennel Club, which registers about 1 million dogs a year, says it has the best data available to rate the popularity of America's 74 million dogs. But it doesn't register mixes or undocumented dogs, which account for half of dogs, by AKC's own estimate.

And it doesn't consider pit bulls to be an official breed at all. So no one knows precisely how many pit bulls there are nationwide -- let alone which breed accounts for the highest number of attacks per dog. "Dog bite statistics are not really statistics, and they do not give an accurate picture of dogs that bite," warns a report from the American Veterinary Medical Association. "Invariably the numbers will show that dogs from popular, large breeds are a problem."

Meanwhile, some insurance agencies have compiled their own lists of vicious breeds, based on claims. Allstate Insurance, for instance, won't offer homeowners insurance to Californians who own any of eight types of dogs: pit bulls (American Staffordshire terriers), akitas, boxers, chow chows, Dobermans, rottweilers, Presa Canarios and wolf hybrids, plus any mixes that include the breeds.

"They are the dogs that generate the most lawsuits," said spokesman Rich Halberg. Nationwide Insurance compiled a similar list, though it doesn't include akitas or boxers.

Company spokesman Joe Case, in Columbus, Ohio, said the carrier consulted the CDC's fatality study, but thought it was critical to consider the insurer's own experience handling reports of other serious dog bites.

"Not every dog attack results in a fatality, but it could result in an insurance claim being filed," said Case. Unlike Allstate, Nationwide will still sell policies to owners whose dog completes the American Kennel Club's "Canine Good Citizen Program," which includes a test to make sure the dog is well behaved.

Delise, who studied dog fatalities dating back to 1965 for her book "Fatal Dog Attacks," has identified numerous patterns in the most serious attacks. She argues that the patterns are more important than the breed.

For instance, dogs kept on chains or for protection posed a much bigger danger than family dogs kept in houses. Most deadly dogs were males. Only a minority of dogs had been spayed or neutered. Many cases involved owners who neglected or abused their dogs, she said.

In one case, the owner had previously been reported for beating his dog with a hammer. Another involved a dog that was starving to death on his chain. By far the majority of those who died from dog attacks were children -- usually unsupervised. A scenario that comes up again and again in the data is the toddler who wanders up to a dog chained in a backyard when no one is watching.

Several infants, left on a floor or bed, have also been killed by a family dog.

In one case, a German shepherd killed an infant by picking it up and carrying it to his family in the living room in what may have been a friendly gesture.

Delise said only a tiny number of dog attack fatality cases -- perhaps two or three each year -- are freak accidents in which a seemingly nice dog goes bad.

Others are either aggressive dogs or abusive owners who create accidents waiting to happen. Or they involve a cascade of mistakes, such as an owner failing to neuter a dog, ignoring a previous aggressive incident and then leaving an unsupervised child with the dog. "Once in a while, the dominoes line up and somebody gets killed," Delise said. "But statistically it's such a small number."
Safety tips

-- Spay/neuter your dog. This reduces aggressive tendencies.

-- Never leave infants or young children alone with a dog.

-- Train and socialize your dog.

-- Seek professional advice if the dog acts aggressive.

-- Advise children to avoid approaching unfamiliar dogs.

More tips are online at www.cdc.gov/ncipc/duip/biteprevention.htm

Source: Centers for Disease Control.