We report on a rare distinctive variant of infiltrating ductal carcinoma characterized by sebaceous differentiation of tumor cells. The neoplasm was identified in a lumpectomy specimen from a 45-year-old woman with extensive metastatic disease. In addition to conventional in situ and invasive ductal components, approximately half of the tumor cells exhibited a phenotype resembling tumors of the sebaceous skin appendage with coarsely vacuolated cytoplasm and peripherally displaced nuclei. The sebaceous moiety was also present in the distant metastatic deposits. There was no evidence of mucin production by tumor cells. Ultrastructurally, empty-appearing non-membrane bound vacuoles attested to the sebaceous cells' lipid content. The immunoprofile of the lesion included positivity for cytokeratin and epithelial membrane antigen. Vimentin, S100 protein and carcinoembryonic antigen were not expressed. Most tumor cell nuclei reacted with antibodies to oestrogen and progesterone receptors but failed to show overexpression of the HER2/neu protein. The MIB-1 labeling index averaged 16%. At variance with sebaceous breast carcinomas on record, the present case is notable for its prolonged clinical course.