Despite dramatic improvements in the survival rate of cancer patients, the incidence of cancer in the United States rose 15 percent between 1973 and 1987, and experts can't explain why.

Cancer cases have risen among blacks, whites, men, women and most age groups, including children.These alarming findings from the National Cancer Institute (NCI) are reported in an article in an issue of American Health.

"The sad truth is we don't really know what's causing a lot of these changes," says Dr. Devra Davis of New York City's Mount Sinai Medical School. Davis, the author of a separate study that found cancer increases in six other industrialized Western nations, believes the causes for most of the increases fit into two categories: cancer-causing chemicals and lifestyle.

"I wouldn't be surprised if some as yet unrecognized aspect of modern industrial life was involved," says Davis, citing asbestos, agricultural pesticides and industrial solvents as possible causes.

Dr. Emily White of the Fred Hutchinson Cancer Research Center in Seattle believes lifestyle plays a far more important role than chemicals in causing cancer. "I know of almost no evidence that environmental toxins cause the increases," she says. Instead, she emphasizes lifestyle factors such as smoking, high-fat diets, excessive drinking and sunbathing.

The deadly skin cancer melanoma recorded the largest rise for any cancer from 1973 to 1987 - 83 percent.

This sharp rise is strongly linked to sun exposure. Some scientists believe the problem will worsen as the earth's ozone layer, which filters cancer-causing ultraviolet rays, continues to thin because of chlorofluorocarbons and other air pollutants.

In tracking long-term cancer trends, NCI found a nearly 500 percent increase in female lung cancer since 1950. Dr. Edward Sondik of NCI's Division of Cancer Prevention and Control said, "That statistic ought to throw people back on their heels."

The reason for the surge among women comes as no surprise. They've taken up cigarette smoking in record numbers over the past 40 years. As a result, lung cancer has now surpassed breast cancer as their leading cause of cancer deaths.

During the 15-year period, childhood cancer rose 6 percent.

"We don't understand why childhood cancer is increasing," Davis says.

With children, Davis adds, lifestyle plays much less of a role in causing cancer than it does among adults. She believes researchers must look for environmental factors such as chemicals, radiation, parents' occupations, smoking before and during pregnancy and perhaps even electromagnetic fields.

Sondik stressed some good news in the generally gloomy report. The incidence of cervical cancer fell 36 percent, which he attributes to increased use of Pap smears. There was also a significant long-term decline in stomach cancer, with new cases decreasing 73 percent since 1950.

"What's absolutely fascinating," Sondik says, "is if you look at statistics for several countries, you see stomach cancer falling, even in Japan and the U.S., where the rate was high. Unfortunately, nobody knows why."

He also noted that despite the increase in cancer cases, deaths from several cancers in people under 65 have fallen significantly between 1973 and 1987. Sondik cited bladder cancer, thyroid cancer, colon and rectal cancer and breast cancer. He attributes these mortality declines to earlier detection and improved diagnostic and treatment techniques.