The most frequent fatal malignant tumors in the Soviet Union are cancers of the lung, stomach, breast, and cervix uteri. From incidence statistics and population estimates provided by the Department of Statistics of the Ministry of Health, age-adjusted incidence rates for the period 1971-87, using the World standard population, have been computed for the USSR as a whole and for each of the 15 Soviet republics. For six republics believed to have the highest quality of data and relatively homogeneous populations, time-trends are examined over the period. There is important geographical variation in the incidence of malignant tumors in the Soviet Union. The differences between high and low incidence areas are four-fold for cancer of the lung and stomach, three-fold for cancer of the breast and two-fold for cancer of the cervix. Overall in the six republics for which time-trends are examined, cancers of the lung and breast have increased, cancers of the stomach and cervix have decreased. There are some variations in specific age groups in some republics.