More than a decade ago, gasoline began to leak from underground storage tanks at four service stations in the Hazleton area of eastern Pennsylvania.

An underground plume of at least 50,000 gallons of gasoline spread beneath more than 350 homes in the middle-class subdivision of Laurel Gardens, sending fumes into the houses through the sewer system.

So far, the federal Environmental Protection Agency has spent about $25 million to clean up the site, and continues to filter groundwater and vapors in the soil to get rid of stubborn traces of gasoline.

Now, a newstudy by the University of Pittsburgh says there are excess leukemia cases in the spill area, and that benzene in the gasoline may be the cause.

The study in the Archives of Environmental Health by Ami Patel and colleagues at the Graduate School of Public Health found that while the overall cancer rate for residents in the spill area is slightly lower than the state average, the leukemia rate is at least 4 ¹/₂ times higher than would be expected.

There were four leukemia cases during the study period among the 663 residents studied, and since then, three more cases have been found, said two of the Pitt researchers, epidemiology professor Evelyn Talbott and senior research specialist Jeanne Zborowski.

That may sound like a small number, but an outside benzene expert, Martyn Smith of the University of California at Berkeley, said it's the proportion that matters.

Dr. Smith noted the study found two cases of acute myelogenous leukemia, the type most closely linked to benzene exposure. The general risk of someone getting that kind of leukemia is 1 in 25,000, he said, so to have two cases in a group of fewer than 700 residents is significant, especially since the people who got leukemia lived close to the highest concentration of spilled gasoline.

The study highlights a chronic problem in the United States -- leaking underground tanks, particularly at service stations.

Even though Congress set up a trust fund 20 years ago to fix leaky tanks, financed by a 0.1 cent tax on each gallon of motor fuel, there is still a huge number of sites that haven't been cleaned up.

Federal figures showed a backlog of nearly 120,000 sites nationwide last year, including more than 4,000 in Pennsylvania, which has the seventh highest number of uncleaned tank leaks in the country.

The study also adds fuel to the continuing debate over how dangerous benzene actually is.

Benzene is a common organic hydrocarbon. It is found in tiny amounts in some foods and in smoke from wood fires and cigarettes. It makes up 1 to 2 percent of gasoline, and more than 15 billion pounds of it is produced each year as an industrial chemical.

There is no doubt that benzene can cause cancer in humans, particularly acute myelogenous leukemia. The question is what level and length of exposure are needed to trigger a malignancy.

Christopher Borgert, head of Applied Pharmacology and Toxicology Inc. in Gainesville, Fla., said the evidence should be interpreted cautiously.

"Just because you have a case of acute myelogenous leukemia and a person exposed to benzene doesn't necessarily mean benzene caused the disease.

"As a scientist I see it as my duty to remain skeptical about everything -- in God we trust; all others bring data."

But Berkeley's Dr. Smith and his colleagues have shown in studies of thousands of Chinese shoe factory workers exposed to benzene that even low levels of the chemical can cause a drop in white blood cell counts and other harmful biological changes.

Because damage from benzene is a gradual process, the Pitt researchers have urged continued monitoring of the health of the residents in the Hazleton spill area.

Pennsylvania Department of Health officials will keep track of cancer cases in the area and continue to offer some blood testing to residents who want it, a department spokesman said.

But it's not clear who might pay for more extensive testing. Attorney Jonathan Miller of Philadelphia recently tried to file a class-action lawsuit that would have required the tank owners to pay for medical monitoring of residents, but a judge ruled that the residents' exposures varied too much to certify them as a class.

There are more than 200 individual lawsuits pending against the tank companies that will take years to resolve, Mr. Miller said.

Dr. Smith thinks that anyone living over a gasoline spill has some risk of health problems from benzene, but he doesn't think more common exposures, such as pumping your own gasoline, are much to worry about.

Others aren't so sure.

Devra Davis, director of the Pitt Cancer Institute's Center for Environmental Oncology, said she remains wary.

"I don't want to create the impression that everyone's going to die of cancer from pumping their own gas," she said. "It's just that it is a risk and people should be aware of that and should stand upwind rather than downwind. You can reduce your exposure 100- to 1,000-fold that way."

The Laurel Gardens residents appear to face a more serious health threat, Dr. Talbott said. "Even though the federal cleanup may have prevented future exposures," she said, "that won't have changed anything for the people who were exposed in the past."

Only long-term health surveillance, she said, will be able to show the ultimate effects of the gasoline spill.