Breast cancer is the most frequent malignant tumor in women, whereas it is rare in men. In our own case series the ratio is 175:1. The present paper deals with an evaluation of clinical and morphological findings from a series of 54 de novo male breast cancers observed in our institution from 1978 to 1996 and a comparative discussion of 528 female breast cancers from the same geographic area. We should like to focus on the following observations: At the time of histopathological diagnosis, male patients with breast cancer were on average 67 (34-87) years old and thus 5 years older than women. Below the age of 40, breast cancer is very rare in men. The lag time between first symptoms and surgery was on average 42 weeks, i.e. twice as long as in women. In the vast majority of cases palpation of a retromamillary nodule was the leading diagnostic symptom. Mamillary secretion appeared to be an early symptom with favorable relation to prognosis by tumor size whereas diffuse breast swelling was an unfavorable late symptom. Bilateral carcinoma and double cancer (breast and prostatic cancer) was observed in one case each. Three patients (3/51 = 6%) had a positive family history (breast cancer in 1st and 2nd degree relatives). The average invasive tumor size was nearly identical with 23 mm (s11.02) in men and 25 mm (s13.48) in women. Men presented more frequently with regional lymph node metastases (53% versus 45%), which tended to develop earlier. pT4 cancers were twice as frequent in men compared to women. In situ cancers were found in 2% (1/54) in men and 4% in women. Similar to females, male breast cancers are predominantly of ductal histological type (NOS-cancers), classical lobular carcinoma with LCIS-components were not observed; special forms (tubular, papillary, mucinous) are slightly more common in men. When reviewing our series, need for revision of the origin of tumor was not found in any of the cases. Metastases of prostatic cancer were never misinterpreted as primary breast cancer. In case of isolated NSE-reaction, cancers with carinoid differentiation pattern are to be found in nearly every second tumor. However, when multiple markers were used (chromogranin A or synaptophysin) only 10% displayed such pattern, which corresponded to a positive hormone receptor status in each case. Quantitative (enzyme immunoassay) and semiquantitative (immunohistochemistry) analysis of steroid hormone receptor status was positive in 86% of 35 cases in men and in 75% in women. In contrast to female breast cancer, hormone status proved to be independent of age in males. The average levels of estrogen and progesterone were higher in men. Overlapping results were found only when cases were compared with postmenopausal women. The Nottingham prognostic index, a product of primary tumor size, axillary lymph node status and grading allows an approximative estimate of the course of the disease; its predictive value is higher than that of isolated tumor markers.