Of 3558 women with cancer of the breast, 129 were found to have bilateral primary cancers. In approximately one quarter (33) of these women the two cancers developed synchronously; in the remainder (96) the second cancer developed metachronously, more than 6 months after the appearance of the first primary tumour. In two thirds (64) of the women in the latter group the second cancer made its appearance within 4 years after the first was detected. The 15-year survival curve for the 129 patients shows a 50% mortality at approximately 10 years, slightly better than that for the total group of 3558. At the time of our study (April 1975) 70 were alive and 59 were dead; 53 died as a result of their breast cancer and 6 from other causes. Only 3 of the 48 women in whom the first cancer developed before the age of 70 years died of causes other than breast cancer, giving a mortality due to breast cancer of 90%. The clinical stage, the recorded histopathology, the type of operation and the duration of time at risk after the first diagnosis did not differ significantly between those who lived and those who died. The authors conclude that age, stage or type of growth, operative therapy or time at risk do not determine the time of death or alter the 90% certainty that death will be due to carcinoma of the breast.