Findings from previous studies suggest that differences in socioeconomic status may be responsible for some, if not all, of the elevated incidence of cancer among blacks as compared with whites. Using incidence data from the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) Program, we tested this hypothesis by correlating black and white cancer incidence rates in three US metropolitan areas between 1978 and 1982 with data from the 1980 census on socioeconomic status within individual census tracts. The study analyzed data on the incidence of cancer at all sites combined (greater than 100 cancer sites) and at seven major sites separately. As in other studies, income and educational levels served as surrogates for socioeconomic status. The present study also used census-tract data on population density as a surrogate factor. Each of these measures of socioeconomic status was analyzed independently. Before correlation with census-tract data, age-adjusted data on cancer incidence showed statistically significant elevated risks among blacks for cancer at all sites combined and at four of the seven separate sites; whites showed an elevated risk for cancer at two sites. Cancer at only one site, the colon, showed no significant association with race. When age-adjusted incidence data were correlated with socioeconomic status, the comparative black-white risks changed: Whites showed an elevated risk of cancer at all sites combined and at three of the seven separate sites; blacks maintained their elevated risk at three sites. These findings suggest that the disproportionate distribution of blacks at lower socioeconomic levels accounts for much of the excess cancer burden among blacks. They also suggest that for both blacks and whites unidentified racial factors, which may be either cultural or genetic and which are not closely linked to socioeconomic status, may play a role in the incidence of some cancers.