The magnitude of the clinical problem of bilateral occurrence of breast cancer is controversial. In order to evaluate clinical and pathologic factors associated with increased risk for metachronous, contralateral breast cancer, we reviewed the records of 187 patients undergoing operative treatment for primary breast cancer by the same surgeon at a single institution. Variables analyzed included age, race, history of tuberculosis or cancer at other sites, family history of breast cancer or cancer at other sites, prior use of estrogens, tobacco, and alcohol, marital status, parity, age at first pregnancy, tumor size and location, histologic diagnosis, degree of cellular differentiation and involvement of axillary lymph nodes. The incidence of metachronous, contralateral breast cancer was 11.8 per cent. The only factor with a statistically significant association with bilateral cancer was histologic diagnosis characterized by multicentricity. Life table survival analysis revealed that the five-year survival following treatment for metachronous, contralateral cancer was 59 per cent. We conclude that bilateral breast cancer is a significant clinical problem, that histology characterized by multicentricity is associated with a higher incidence of bilateral breast cancer, and that favorable survival justifies an aggressive approach.