Using the monoclonal antibody (MAb) MRK20 the immunohistochemical expression of the 85-kd membrane protein (MP)/cell adhesion molecule (CAM) CD36 was investigated in tumors from 41 patients with untreated breast adenocarcinoma. DNA ploidy as measured by flow cytometry and estrogen (ER) and progesterone receptor (PgR) tumor contents also were determined. Immunohistochemistry on frozen tissue showed expression of 85-kd MP localized to the carcinoma cells in tumors from 16 patients (39%). These samples also displayed variable staining of the capillary endothelium. Twenty-one samples (51%) showed 85-kd MP expression in small blood vessels. Thus, a total of 37 of 41 tumors (90%) were MAb MRK20 positive in carcinoma and/or mesenchymal cells. Expression of 85-kd MP in carcinoma cells was negatively associated with expression of ER (P < .05) and PgR (P < .01) receptors and positively associated with nondiploid tumors (P < 0.05) and tumor grade (P < .05). Axillary lymph node status, tumor size, and (patient) age were not associated with expression of 85-kd MP. In cytological smears obtained by fine-needle aspiration (FNA) cytology from 19 of the samples, expression of 85-kd MP in carcinoma cells showed a good correlation with tumor cell positivity in the corresponding histological section. Expression of 85-kd MP in breast carcinoma cells thus was associated with some recognized adverse prognostic factors. Future studies of its pathophysiological and prognostic roles in breast cancer seem warranted.