Preoperative aspiration biopsy of a breast tumor in a 20-year-old male resulted in a diagnosis of secretory breast carcinoma. The material obtained from aspiration biopsy was also used for electron microscopic, immunocytochemical and flow cytometric analysis. The characteristic light microscopic features were numerous large secretory vacuoles containing proteinaceous material, which was also seen in the background of the smear. Electron microscopy revealed numerous intracellular lumina and some intercellular ones filled with secretory material as well as membrane-bound vacuoles. Immunoreactivity was positive for cytokeratin, epithelial membrane antigen, carcinoembryonic antigen, alpha-lactalbumin and vimentin. Flow cytometric analysis showed the tumor to be diploid. The affected breast and tumor were resected, and the cytologic diagnosis was confirmed histopathologically. The cytomorphologic features of secretory breast carcinoma seem to be alike in men and women and are sufficiently characteristic to permit a definitive preoperative diagnosis from fine needle aspiration biopsy smears. The differential diagnosis with other tumors is discussed briefly.