The incidence of second primary cancers was determined in 238 patients with carcinoma of the breast treated by radical mastectomy and not irradiated, and in 846 patients treated by radical mastectomy and given prophylactic postoperative radiotherapy. Patients who developed local or distant metastases were excluded. The average time of observation was 86 months. Second cancers were expressed as the number per 1,000 patient-months at risk for each interval of ten years. The incidence was the same in those who did or did not receive prophylactic postoperative radiotherapy. Fifty-seven percent of the second cancers occurred in the contralateral breast. The incidence was 8.8 per 1,000 patients per year. The results show that prophylactic postoperative radiotherapy does not affect the incidence of second primary cancers in patients with carcinoma of the breast.