Carleton and Christina Giles and their 4-year-old daughter, Courtney, pay a visit to Cezanne, a Rottweiler they had to give up at the insistence of their insurer, which blacklists certain breeds of dog.

The big Rottweiler was an awesome sight: glistening black fur, 135 pounds of muscle and nearly 6 feet tall when it rose up on its hind legs. Strangers scattered when they saw it coming.
But Carleton Giles and his wife, Christina, always kept the dog, Cezanne, on a leash, and neighbors on their quiet street in Columbia, S.C., say their pet was invariably gentle and even cuddly.
"He's a big baby," said Woody Freeman, who lives next door to the Gileses. "The big joke was the worst he could do, as big as he was, would be to step on your foot while you were playing with him. He is a very friendly dog."

But Nationwide, which for seven years has sold the Gileses insurance on their home and two cars, was taking no chances. It considers Rottweilers, Dobermans, pit bulls, Presa Canarios, chows and wolf hybrids dangerous, and simply refuses to insure homes where they are kept. Some other insurers also have blacklists of breeds, including Akitas and huskies, which they compile on the basis of both reputation and dog-attack statistics from the Centers for Disease Control and Prevention. Without the insurance, it is almost impossible to get or keep a mortgage.

The danger of big, powerful and often aggressive dogs has been underscored by the fatal mauling of a San Francisco woman in the doorway of her apartment by a neighboring couple's 120-pound Presa Canario. Just last week, one of the dog's owners was convicted of murder, the other of involuntary manslaughter. The day before, a Wisconsin couple were charged with homicide because their six Rottweilers had killed a 10-year-old girl who was playing in their home with their daughter.

But insurers began focusing on the problem of dangerous dogs long ago. Since the mid-1990's, with the growing popularity of aggressive breeds as a result of security concerns, dog attacks have become the biggest single cause of home policy claims, with costs running to more than $300 million a year, said Robert P. Hartwig, chief economist of the Insurance Information Institute, a trade group.

State Farm, Allstate and many other insurers do not have blacklists. Instead, their agents make a judgment call on each dog, regardless of breed, as Nationwide and some other insurers do in cases of breeds that are not on their lists. "If the dog is growling or aggressive, that's pretty much a tip-off that you could have a problem dog," said Dan Hattaway, a specialist on home insurance at State Farm. Mr. Hattaway said State Farm did not think blacklisting of breeds was an effective way to screen for danger. "Simply by naming a breed," he said, "people are screening out a fair percentage of good dogs. Just because someone is a Rottweiler fancier doesn't mean they have a dangerous or aggressive dog."

When Nationwide discovered the Gileses' Rottweiler, during a visit to check possible damage to their roof, it issued an ultimatum: get rid of the dog or give up coverage on the three-bedroom ranch-style home. For the Gileses, the choice was heartbreaking. They considered Cezanne a part of the family, a big brother and playmate for their 4-year-old daughter, Courtney.
For Nationwide, it was a straightforward business decision.

"We believe these breeds are capable of inflicting damage or causing a loss that is far outside the norm," said Scott Mallasee, a specialist in home insurance at Nationwide.
J. Robert Hunter, director of insurance for the Consumer Federation of America, said it made sense for insurance companies to deny coverage because of a dangerous dog. "If you had a dynamite factory in your basement, it would be silly for an insurance company to sell you coverage," Mr. Hunter said. "Likewise if you have a dog that is biting everybody or exhibiting menacing behavior."

But he and others said they objected to denial of coverage on the basis of breed alone.
"Dogs, like people," Mr. Hunter said, "have a wide range of behavioral patterns and personalities."
Sometimes, veterinarians say, the most troublesome dogs are those that might be least suspected.
"The most vicious dogs I see are Chihuahuas," said Dr. Kevin Blaes, chief of staff at Banfield, the Pet Hospital of Columbia. "They don't do a lot of damage, but some of them just keep gnawing and biting. And Dalmatians are becoming very aggressive. You don't expect that."
Unlikely though it may seem, tiny dogs sometimes kill. A little over a year ago, for example, a Pomeranian, one of the smallest breeds, jumped onto a bed and killed an infant in a home near Los Angeles.

At Nationwide, Mr. Mallasee defended his company's policy of discriminating by breed.
"We just don't have the luxury of getting to know the dog to the point where we would believe that this dog does not pose the risk that the breed would indicate," he said. Nationwide and some other companies also refuse to sell or renew coverage when a dog has attacked someone. But State Farm, the largest insurer of homes, and Allstate, the second-largest, say they will consider selling or continuing coverage even after a dog bite.

"We have to look at the situation," said Michael Trevino, a spokesman for Allstate. "Is the dog really vicious? Is he going to bite again? Or was it just an accident?"
Even at companies that do not blacklist, though, individual agents are often wary. Mr. Giles, a graphics designer, said he had tried to change insurance companies. But because of his big dog, he said, he could not find an agent who would sell him coverage. One who offered coverage, he said, wanted to triple his premium.

Finally he gave Cezanne to Sarah Elkin, a local dog groomer. She agreed that if the Gileses could ever find coverage that would include Cezanne, she would give him back. She lives out in the country in a home without a mortgage. She has no insurance on the house. So the views of the insurance companies are of no concern to her.