AIMS--To investigate the effect of combination endocrine treatment (CET) or luteinising hormone releasing hormone agonist and flutamide on non-neoplastic prostate, prostatic intraepithelial neoplasia, and prostatic adenocarcinoma. METHODS--The morphology, including the mitotic activity, of 12 radical prostatectomies from patients with prostatic adenocarcinoma pretreated for three months with CET was evaluated in haematoxylin and eosin stained sections and compared with an untreated age and stage matched control group. RESULTS--A differential effect on the non-neoplastic prostate was observed. In fact, the transition zone of the treated prostate showed simplification of the glandular lobules: the ducts and acini were small without undulations of the epithelial border and with a prominent basal cell layer. Within the peripheral zone there was inconspicuous branching of the ducts and acini which looked dilatated and lined by flattened atrophic epithelium. Prostatic intraepithelial neoplasia occurred in scattered ducts and acini in the peripheral zone of 10 of the 12 patients. The epithelial cell lining showed a prominent basal cell layer. A certain degree of secretory cell type stratification was always present. However, crowding was less evident than in the untreated prostate because of cytoplasmic clearing and enlargement as a result of coalescence of vacuoles. The treated adenocarcinomas had neoplastic acini which looked small and shrunken, and areas of individual infiltrating tumour cells separated by abundant interglandular connective tissue. The secretory cells of the nonneoplastic, prostatic intraepithelial neoplasia, and prostatic adenocarcinoma lesions had inconspicuous nucleoli, nuclear shrinkage, chromatin condensation, and cytoplasmic clearing. Apoptotic bodies were easily identifiable in all the cell layers. The lumina were rich in macrophages, sloughed secretory cells with degenerative features, and apoptotic bodies. Mitoses were not observed in any of the treated non-neoplastic prostate, prostatic intraepithelial neoplasia, or prostatic adenocarcinomas, whereas the mitotic frequency increased from non-neoplastic prostate through prostatic intraepithelial neoplasia up to prostatic adenocarcinomas in the untreated specimens. CONCLUSIONS--CET before radical prostatectomy causes regressive epithelial changes together with enhanced apoptosis and blocked mitotic activity.