1,271 patients with breast cancer treated at the Institut Gustave Roussy between 1967 and 1972 and with a minimum follow-up of 10 years, have been studied in order to analyse the risk factors for bilateralization. Patients with metastases at presentation (160) who have an incidence of bilateralization at two years of 20% have been excluded since the contralateral tumor is regarded as part of the metastatic process. For 1,111 patients, non-metastatic at presentation, the following factors have been studied: age, T-stage, N-stage, tumor grade, tumor growth rate (doubling tumor size in less than six months) and the presence of inflammatory signs. Of these factors, only advanced T stage, fixed axillary lymphadenopathy and the presence of inflammatory sign were associated with a significantly increased risk of bilateralization. For patients presenting with T1 tumors the incidence of bilateralization is 19% at 10 years but this is probably because relatively more of these patients lived long enough to develop a second cancer. A more detailed histopathological study was performed on 682 patients whose tumors were operable at presentation and for whom the following histological characteristics are known: type, grading (Scarff and Bloom), number of axillary nodes involved by tumor and anatomical size of the tumor. None of these characteristics was found to increase the risk of bilateralization. Comparing the two breast tumors (and excluding those with a diffuse infiltration in either breast) in 74 patients in whom the exact tumor site was known, in only 7, was the second tumor a "mirror-image" of the first. Overall, MO patients with bilateral tumors have a decreased survival compared with those with unilateral tumors. For those patients operable at presentation, the 10 year survival is 51% and 63%. The conclusions of this study are that there are two populations of patients with bilateral breast cancer: Those in whom the controlateral tumor is part of a generalized metastatic process and occurs particularly in those with a poor prognosis (metastases at presentation, inflammatory carcinomas, fixed lymphadenopathy). Those in whom there is a relatively long interval (5-10 years) between the development of the two tumors who have not any metastases. This population particularly comprises patients with T1 tumors thus for whom continuing clinical and mammographic follow-up is justified.