From 1970 to 1992, 31 pure ductal carcinoma in situ (DCIS) of the male breast treated in 19 French Regional Cancer Centres were reviewed. They represent 5% of all breast cancers treated in men in the same period. The median age was 58 years, but 6 patients were younger than 40 years. TNM classification (UICC, 1978) showed 12 T0 (discovered only by bloody nipple discharge), 10 T1, 5 T2 and four unclassified tumours (Tx). 11 patients (35.5%) had clinical gynecomastia, and three (10%) had a family history of breast cancer. 6 patients underwent lumpectomy, and 25 mastectomy. Axillary dissection was performed in 19 cases. 6 cases received postoperative irradiation. 15 out of 31 lesions were of the papillary subtype, pure or associated with a cribriform component. The size of the 12 measured lesions varied from 3 to 45 mm. All lymph nodes sampled were negative. With a median follow-up of 83 months, 4 patients (13%) presented a local relapse (LR), respectively, at 12, 27, 36 and 55 months. 3 of these patients had been initially treated by lumpectomy. In one case LR was still in situ, but already infiltrating in the 3 others. Radical salvage surgery was performed in 3 cases, but one patient developed metastases and died 30 months later. The last patient was treated by multiple local excisions and tamoxifen. One 43-year-old patient developed a contralateral DCIS and three others developed a metachronous cancer. The aetiology and risk factors of male breast cancer remain unknown. Gynecomastia, which implies an imbalance between androgen and oestrogen, may be a predisposing factor. As in women, DCIS in the male breast has a good prognosis. Total mastectomy without axillary dissection is the basic treatment. Frequently, the first symptom is a bloody nipple discharge. The age of occurrence is younger than for infiltrating carcinoma, suggesting that DCIS is the first step in the development of breast cancer.