The presence of neuroendocrine differentiation, as expressed by cellular chromogranin immunoreactivity, was investigated in paraffin-embedded tissue material from 51 consecutive cases of male breast carcinoma. From six of these cases electron microscopic studies were included. Chromogranin-immunoreactive cells were present in solid cords and delineated tubular structures. Ultrastructurally, dense core secretory granules could be detected. The expression of neuroendocrine differentiation was 45%, which is between two and eight times higher than reported for female breast carcinomas by other investigators. The present findings suggest that male breast carcinoma is an exclusive tumour disease showing both similarities and discrepancies when compared to its female counterpart.