Ovarian theca cells propagated from patients with polycystic ovary syndrome (PCOS) convert steroid precursors into T more efficiently than normal theca cells. To identify the basis for increased T production by PCOS theca cells, we examined type I-V 17 beta-hydroxysteroid dehydrogenase (17 beta HSD) isoform expression in long-term cultures of theca and granulosa cells isolated from normal and PCOS ovaries. RT-PCR analysis demonstrated that theca cells express type V 17 beta HSD a member of the aldo-keto reductase (AKR) superfamily (17 beta HSDV, AKR1C3), whereas expression of type I, II, and IV 17 beta HSD, which are members of the short-chain dehydrogenase/reductase superfamily, was limited to granulosa cells. Type III 17 beta HSD, the testicular isoform, was not detected in either granulosa or theca cells. Northern and real-time PCR analyses demonstrated that 17 beta HSDV transcripts were not significantly increased in PCOS theca cells compared with normal theca cells. RT-PCR analysis revealed that theca cells also express another AKR, 20 alpha HSD (AKR1C1). Both basal and forskolin-stimulated 20 alpha HSD mRNA levels were increased in PCOS theca cells compared with normal theca cells. However, 17 beta HSD enzyme activity per theca cell was not significantly increased in PCOS, suggesting that neither AKR1C3 nor AKR1C1 contributes to the formation of T in this condition. In contrast, 17 alpha-hydroxylase/C17,20 lyase and 3 beta HSD enzyme activities were elevated in PCOS theca cells, driving increased production of T precursors. These findings indicate that 1) increased T production in PCOS theca cells does not result from dysregulation of "androgenic" 17 beta HSD activity or altered expression of AKRs that may express 17 beta HSD activity; and 2) increased synthesis of T precursors is the primary factor driving enhanced T secretion in PCOS.