We evaluated the immunolocalization of the steroidogenic enzymes involved in the production of ovarian steroids, including the cholesterol side-chain cleavage enzyme (P450scc), 3beta-hydroxysteroid dehydrogenase (HSD), 17alpha-hydroxylase (P450c17) and aromatase (P450arom), oestrogen receptor (ER) and androgen receptor (AR), a steroidogenic transcription factor. Ad4-binding protein (Ad4BP) and a cell cycle-related nuclear antigen, Ki67, in five patients with polycystic ovarian syndrome (PCOS). Results were compared with those from normal cycling human ovaries to study in situ ovarian steroidogenesis and cell proliferation in polycystic ovaries (PCO). We classifed the follicles morphologically according to the development of granulosa types: type A, more than four layers (n = 7); type B, one to three layers (n = 11); and type C, theca interna cells only (n = 21). ER and P450arom were not observed in any of the follicles examined. In type A follicles, P450scc, 3beta-HSD, P450c17, AR and Ad4BP were observed in theca cells in all seven follicles examined, but the granulosa cells were positive only for Ad4BP (4/7) and AR (7/7). These immunohistolocalization patterns resembled those in non-selected antral follicles of normally cycling human ovaries. In theca cells from types B and C follicles, follicles positive for the steroidogenic enzymes, AR and Ad4BP were decreased in number. There were no significant differences between types A and B PCO follicles in the Ki67 labelling index of granulosa or theca cells, and between PCO and antral follicles from normally cycling human ovaries. Data demonstrate that the follicles of PCO are by no means atretic and are actively involved in both steroidogenesis and cell proliferation. The absence of ER and aromatase expression in the granulosa cells of PCO may be important in abnormal follicular development in patients with PCOS.