The number of new cases of most major forms of cancer continues to increase about one percent each year, according to a report issued yesterday by the National Cancer Institute.

Scientists said they are unable to explain the steady rise. While smoking continues to be the most significant cause of cancer, it does not explain increases in other cancer cases. Experts are only able to offer general ideas about why cancers other than lung cancer are on the rise. They cite exposure to industrial chemicals at work, industrial wastes, exhausts of diesel engines, radon, and vague links between diet and cancer.

Both the incidence and the death rate from breast cancer is on the rise, although the overall death rate from cancer is down slightly when lung cancer is excluded, according to the annual NCI statistical report.

When lung cancer is included, however, overall cancer deaths continue to rise, raising questions about whether the cancer institute will be able to reach its goal of cutting the death rate in half by the year 2000.

Although millions of Americans have quit smoking since the surgeon general's first report on smoking in 1964, the number of smokers appears to have leveled off for white men. Each year fewer black men and women are smokers, but the number of smokers among white women is increasing slightly.

"The smoking battle is by no means won," said Edward Sondik, chief of NCI's Division of Cancer Prevention and Control and principal author of the report.

Among white men, the number of new lung cancer cases dropped in 1985 to its lowest level since 1977. The number of new cases among black men was at its lowest level since 1982. For both black and white women, however, the number of cases continues to climb. The lung cancer death rate for men has been stable. It is rising among women.

For breast cancer, the number of new cases went up 1.1 percent a year for white women and 1.3 percent a year for black women between 1976 and 1985. No one knows why the number of new cases continues to rise, Sondik said. The mortality rate is rising too, he said, indicating that there has been a real increase in the disease, not just an increase in detection.

For white women younger than age 50, the death rate rose 5 percent during 1984 and 1985. Before that, breast cancer mortality had been declining steadily since 1973. For white women older than age 50, breast cancer deaths continue to rise.

The data suggest that breast screening programs using mammograms have not been working because not enough women use the procedure, Sondik said. Although record numbers of women get mammograms -- low energy X-rays used to spot breast tumors -- only 16 percent of the women older than 50 get one each year.

If mammography was used by all women at risk, "we could reduce the mortality of breast cancer in women over 50 by 30 percent," Sondik said.

For the first time, NCI combined a number of studies to provide trends over the 35-year period from 1950 to 1985. From 1950 to the mid-1970s, overall cancer death rates went down steadily. After that death rates leveled off.

The statistics show that new treatments have improved the prognosis for some cancers but for others aggressive screening and treatment techniques have failed.

For example, cancer of the testis has risen 85 percent over the 35 year period, but because of effective chemotherapy, the death rate has fallen 60 percent. Hodgkin's diseases, a blood cancer, is up 24 percent over the same period, but mortality is down 61 percent. For some diseases, such as cervical cancer, both the incidence and the mortality is down. Cervical cancer may be declining because Pap smears are detecting precancerous cells that are aggressively treated before cancer actually develops. More than half of women over age 20 report having had a Pap smear within the past year. But some forms of the disease continue to rise unabated. Melanoma of the skin, a disease closely associated with exposure to the sun, is up 242 percent in the last 35 years. The death rate from melanoma rose 150 percent over that period.