Half of all Americans diagnosed with cancer this year are expected to survive at least five years, the National Cancer Institute reported in December. For many cancers, this survival rate amounts to a cure.

But the news is less hopeful for black Americans. Their five-year survival rate is much lower -- about 38 percent, or the same as the cancer survival rate for all Americans in 1950. Blacks are more likely than whites to get -- and die from -- most types of cancer.

A recent study of cancer rates in 10 major metropolitan areas found that Washington has one of the lowest rates for whites and the highest cancer rate for blacks. Blacks in the District had higher rates than whites for cancer of the cervix, colon, esophagus, liver, lung, pancreas, prostate, rectum and stomach.

The disproportionately high rate of cancer in black Americans results not from genetic differences but from socioeconomic and dietary factors and a higher rate of smoking, particularly in men, said Dr. LaSalle Leffall Jr., a leading cancer surgeon at Howard University.

"We have found no evidence that it is genetic," said Leffall, chairman of the department of surgery at Howard and former president of the American Cancer Society, at a breakfast meeting last week for colleagues and the press.

A high-fat, low-fiber diet is associated with three of the four leading cancers -- breast, colon-rectum and prostate -- and the other, lung cancer, is mainly caused by smoking, he said.

A study of prostate cancer by Howard University and the University of Ibadan in Nigeria recently compared black American patients with black African patients. Black Africans have a low rate of prostate cancer, while black Americans have one of the highest rates in the world.

The only significant difference between the two groups, Leffall said, was diet. The black Americans tended to consume much more fat and much less fiber.

Black Americans, particularly in urban areas, are more likely than whites to lack health insurance, education and easy access to health care. That increases their risk of death from cancer, because many cancers are successfully treated only if detected early and treated immediately.

Four factors in particular are responsible for the black-white gap in cancer incidence and mortality, Leffall said:

*Smoking. The biggest reason for the higher cancer death rate in blacks is that black men are more likely to smoke than white men. And black women over 40, by turning to smoking in greater numbers, Leffall said, are "rapidly gaining a kind of parity they don't want."

*Diet. Although dietary differences and their effects are difficult to measure between individuals, they show up clearly in studies of large populations. Such studies suggest that blacks, as a group, tend to follow a diet relatively high in fat and low in fiber -- thereby increasing the risk of cancer.

*Alcoholism. Black American men age 35 to 44 are 10 times as likely as whites to get cancer of the esophagus, a leading cause of which is alcohol consumption.

*Occupational hazards. Although worksite dangers and air pollution account for a small percentage of cancers, Leffall said, blacks workers are more likely to have jobs that expose them to toxic air and chemicals.

"If there's a dirty job -- one where there's a lot of fumes and toxic substances -- you're going to find a black American there more than you'll find a white American," he said.

Studies show that poverty and lack of education boosts cancer mortality rates for poor whites as well as poor blacks, Leffall said. "Those of us in this room who call ourselves middle class," he said, "will have the same rate as the white majority.

"Race seems to play very little role in cancer rates unless you're poor."