The authors conducted this study to determine the probability of a second primary developing in the contralateral breast of a patient, based on her present age, age at her first breast primary, and family history of breast cancer in her mother, sister, or second-degree relative. The study involved 556 patients. With premenopausal diagnosis of the first primary, the probabilities to the 19th year for the three types of family histories ranged from 35% to 38%; with postmenopausal diagnosis, they ranged from 11% to 26%. The probability for all familial patients was 28%. This contrasts with a probability of 13% reported for a general series of patients. An early age at diagnosis and family history of the disease thus have important enhancing effects on the development of second primaries. This information could be useful to physicians in deciding how best to manage the treatment of their patients. Cancer 56: 2092-2098, 1985.