Breast cancer laterality of over 250,000 cases from the Surveillance, Epidemiology, and End Results (SEER) program in the United States was studied in relation to gender, race, tumor stage, histology, age at diagnosis, year of diagnosis, estrogen receptor status, and marital status. The data, which include all invasive and in situ breast cancer cases in the SEER program during the years 1973-92, confirm results from other studies of an overall five percent excess of left-sided disease in women. The excess occurs for all races and stages of disease, and for invasive disease, the excess increases with age. There was no significant variation in the laterality of invasive disease over time, though for in situ tumors, the left-sided excess was significantly greater during the years 1978-82 than in other periods. No excess of left-sided breast cancer was observed among men. There is no evidence that detection bias plays a major role, and although the left breast is slightly larger, on average, than the right, there is little evidence that breast size is associated with breast cancer risk. The reason for the left-sided excess among women remains unclear.