The etiology of breast cancer in men is not well understood. We assessed the risk of subsequent cancers among all 1788 men diagnosed with a first primary breast cancer from 1973 through 1996 who were registered with the Surveillance, Epidemiology, and End Results (SEER) Program. Although the overall subsequent cancer risk in men was not increased (standardized incidence ratio [SIR] = 0.99, 95% confidence interval [CI] = 0.86 to 1.1), the risk of contralateral second breast cancer was strongly elevated (12 cases; SIR = 30, 95% CI = 15 to 52). The risk was higher for men diagnosed with their first breast cancer before age 50 years than for older men. There were no major differences in the risk of contralateral breast cancer associated with different treatments received for the first breast cancer. The relative risk of second breast cancer was substantially higher among men than among women with breast cancer, but the absolute excess risk was lower. We conclude that men diagnosed with breast cancer are at high risk of contralateral breast cancer.