Immunohistochemical localization of 17 alpha-hydroxylase/C17-20 lyase (P-450(17 alpha,lyase)) and aromatase cytochrome P-450 (P-450arom) in polycystic ovary (PCO) syndrome was studied using specific polyclonal antibodies which had been raised against the corresponding enzymes. In the majority of follicles that were atretic and smaller than 7 mm in diameter, theca interna cells showed high P-450(17 alpha,lyase) immunoreaction, while small numbers of granulosa cells showed little P-450arom immunoreaction. In some atretic follicles that were larger than 11 mm in diameter, the hyperplastic theca interna cell layer showed high immunoreaction to P-450(17 alpha,lyase), while the poorly proliferated granulosa cell layer showed a mixture of weak and negative immunoreaction to P-450arom. No immunoreaction to P-450(17 alpha,lyase) or P-450arom was recognized in PCO stroma. These findings suggest that the theca interna cells and the granulosa cells from PCOs show abnormal steroidogenic function, while the localization of P-450(17 alpha,lyase) and P-450arom in PCOs was essentially identical to that in the normal ovary. Theca interna cells in PCO atretic follicles are the main site of excess androgen production.