BACKGROUND. The Soviet Union offers a unique frame in which to study geographic variation in cancer incidence because of its uniform registration system of all newly diagnosed cancer cases throughout its 15 republics and 162 oblasts (administrative units). Variation in cancer rates is stronger when examined by oblasts than it is when examined by republics. In 1986, the age-standardized all-site cancer incidence rate for both sexes, in the Soviet republic having the highest overall rate (Russian Soviet Federative Socialist Republic for males, Lithuania for females) was about twice that of the Soviet republic with the lowest all-site cancer rate (Georgia). Within Soviet oblasts (data are from 1979), the ratio of the highest to the lowest rates was about 4 for all sites, 144 for male oesophageal cancer, and 303 for female oesophageal cancer. The excess incidence in the entire USSR compared with the republic having the lowest all-site incidence rate was 59% for males and 49% for females. CONCLUSION. Although the observed differences between republics or between oblasts are partially due to uneven quality of data recording or to statistical variability related to population sizes, there is a strong geographic variation of cancer incidence rates in the USSR that suggests a potentially important role of environmental factors in cancer etiology.