BACKGROUND: The present study was designed to evaluate whether the alteration of aquaporin-9 (AQP-9) expression in granulosa cells (GCs) of patients with polycystic ovary syndrome (PCOS) was associated with the hyperandrogenism in follicular fluid (FF). METHODS: We recruited infertile women with PCOS (n = 14) and infertile women with tubal blockage (controls, n = 31) for this study. We examined total testosterone (TT), free androgen index (FAI), sex hormone-binding globulin (SHBG), FSH, LH and estradiol in FF. Real-time PCR and western blotting were performed to assess AQP-9 expression in GCs, including effects of dihydrotestosterone (DHT) in vitro. RESULTS: AQP-9 protein was localized in the nucleus, cytoplasm and cell membrane of the human GCs. The TT, FAI and LH levels were all higher, and SHBG levels lower, in the FF of women with PCOS versus controls (P = 0.0145, 0.0001, 0.0191, 0.0001, respectively). AQP-9 mRNA level in GCs of patients with PCOS was tightly correlated with the TT, SHBG levels and FAI in FF (P = 0.0020, 0.0001, 0.0020, respectively). In vitro, DHT (10(-9) mol/l) decreased AQP-9 mRNA (lowest at 12 h) and protein levels in control GCs (P = 0.0005, 0.0247, respectively). The inhibitory effect of DHT on AQP-9 mRNA was attenuated by LY294002, a phosphatidylinositol 3-kinase (PI3K) inhibitor (P = 0.0013). Fifty micromolar 4-(hydroxymercuri) benzoic acid sodium salt (PMB) and 10(-9) mol/l DHT blunted the swelling of GCs in hypotonic medium, respectively (P = 0.0350, 0.0027). CONCLUSION: Hyperandrogenism in FF of women with PCOS inhibited AQP-9 in GCs through the PI3K pathway.