In literature data, an uncertainty exists whether occurrence of bilateral breast cancer decreases the survival probability of affected patients. Therefore, we analyzed the medical records of 498 postoperatively irradiated (1977-1982) female breast cancer patients (T1-4,N0-3,M0). In the follow-up time, in 36 patients a bilateral breast carcinoma treated by surgery with or without radiotherapy was found. The 10-year overall survival rates were 54% in patients who had unilateral disease, compared with 56% in bilateral carcinoma patients, respectively. The incidence of metastasis did not differ between both groups: 24.2% versus 38.8%. Eleven percent of unilateral cancers recurred; in the other group, local failure of the first and second tumor was observed in 19.4% and 11.1%, respectively. We conclude that the occurrence of bilateral breast cancer has no significant impact on survival, although the development of local failures and metastases seems to be more frequent. The therapeutic strategy in bilateral carcinoma should resemble the treatment procedure in unilaterally affected patients.