Although apocrine carcinoma is a distinct histologic entity, there is no immunohistochemical marker to confirm apocrine differentiation with high sensitivity and specificity, and its differential cytologic characteristics are still not fully clarified. Despite the foamy cytoplasm of some apocrine carcinomas and the existence of lipid in the normal apocrine gland, intracytoplasmic lipid in apocrine carcinomas has not been fully explored. By using immunohistochemistry for adipophilin, which is a specific marker of lipid accumulation that can be applied to paraffin sections, we examined intracytoplasmic lipid in apocrine carcinomas. Twenty-four of 26 (92%) apocrine carcinomas and 38 of 116 (33%) nonapocrine carcinomas contained intracytoplasmic lipid. The frequency of adipophilin-positive cases was significantly higher in apocrine carcinomas compared with nonapocrine carcinomas (P<0.01). The positive cell rate per tumor ranged from 10% to 70% (mean, 29%) for apocrine carcinomas. The staining density was heterogeneous from cell to cell. There was no difference in the staining pattern of adipophilin between apocrine ductal carcinoma in situ and invasive apocrine carcinoma or between eosinophilic cells and foamy cells. Sporadic or mosaic distribution of adipophilin-positive cells throughout the tumor and microvesicular or fine granular cytoplasmic staining with heterogeneous density were characteristic features of apocrine carcinoma. Although intracytoplasmic lipid was identified in most apocrine carcinomas, none of the apocrine carcinomas contained prominent intracytoplasmic lipid in >90% of the tumor cells; thus, the criteria for lipid-rich carcinoma was not fulfilled. However, the immunohistochemical study suggests that lipid-rich carcinomas are closely related to apocrine carcinomas.