A young woman with typical polycystic ovary syndrome (PCO) underwent laparotomy for moderately differentiated endometrial cancer. Specimens from the hyperplastic thecal and stromal tissue of the ovaries were incubated for 2 hours in the presence or absence of hCG, 100 IU/ml. Following incubation the tissue content of cyclic AMP and the amounts of progesterone (P), androstenedione (A), testosterone (T) and estradiol-17 beta (E2) in the incubation medium were analysed. For comparison, thecal cells from normal ovaries of regularly menstruating women were incubated under identical conditions. In vivo, the PCO ovaries secreted several-fold greater amounts of T than normal ovaries. In vitro, the thecal cells were much more active, steroidogenically, than the stromal cells of the PCO ovary. Furthermore, the hyperplastic thecal cells of the PCO ovary produced several-fold greater amounts of androgens, and appeared more sensitive to stimulation with hCG, as compared with thecal cells from normal ovaries. The results indicate that in women with PCO associated with endometrial cancer the hyperplastic thecal cells are a significant site of abnormal androgen production and abnormal sensitivity to gonadotropin.