A prospective study was carried out, between January 1980 and March 1982, of all women who presented to the Breast Cancer Unit, Guy's Hospital, with a second primary breast cancer. The percentage of new simultaneous tumours detected clinically or by mammography was approximately 3 per cent. Routine mammographic screening of the contralateral breasts of patients with unilateral disease increased the rate of detection, fivefold, of simultaneous bilateral cancers. The incidence of nonsimultaneous bilateral disease was 7·6 second cancers per thousand patients at risk per year. The annual rate of occurrence of second primary cancers was a constant event and showed no trend either to increase or to decrease with follow-up. However, the risk of a nonsimultaneous second cancer was strongly correlated with age at first primary. Women who developed their first breast cancers under the age of 40 years had three times the risk of developing a second breast cancer compared with those who developed their first cancer after the age of 40. The risk of a second nonsimultaneous primary was 5·9 times that of the risk of occurrence of cancer in the first breast in the general female population.