Two cases of carcinoma in situ of the male breast are described: in one patient it arose as a complication of gynecomastia; in the other, it occurred 'de novo' and proceeded to infiltration with widespread metastases. We reviewed 233 cases diagnosed clinically and histopathologically as gynecomastia to see if we could discover other instances of malignant transformation but none was found. Thirty-two cases of infiltrating male breast cancer were also examined. In 11 there was associated intraduct change. In only one of the 32 cases was there evidence of associated microscopic gynecomastia.