To determine the prevalence of polycystic ovaries (PCO) and polycystic ovarian syndrome (PCOS) in lesbian women compared with heterosexual women undergoing fertility treatment. A prospective observational study. The London Womens clinic and The Hallam Medical Center. Tertiary referral fertility setup. Six hundred eighteen women undergoing ovarian stimulation with or without IUI treatment between November 2001 and January 2003. Of these, 254 were selfidentified as lesbians and 364 were heterosexual women. Baseline pelvic ultrasound examination and blood tests conducted to measure biochemical parameters such as FSH, LH, E2, PRL, T, androstenedione (A), sex hormonebinding globulin (SHBG), and DHEAS were performed between day 2 and 3 of each womans menstrual cycle. Tubal patency tests were performed by hysterosalpingography or laparoscopy. Biochemical parameters. Eighty percent of lesbian women, compared with 32%of the heterosexual women, had PCO on pelvic ultrasound examination. Thirtyeight percent of lesbian women, compared with 14%of heterosexual women, had PCOS. There were no significant differences in the androgen concentrations between lesbian and heterosexual women with normal ovaries. However, lesbian women with PCO and PCOS had significantly higher androgen concentrations compared with heterosexual women with PCO and PCOS. Tubal disease was as common in lesbian women as in heterosexual women. There is a significantly higher prevalence of PCO and PCOS in lesbian compared with heterosexual women. Lesbian women with either PCO or PCOS had more pronounced hyperandrogenism than did heterosexual women with either PCO or PCOS. To determine the prevalence of polycystic ovaries (PCO) and polycystic ovarian syndrome (PCOS) in lesbian women compared with heterosexual women undergoing fertility treatment. A prospective observational study. The London Women s clinic and The Hallam Medical Center. Tertiary referral fertility setup. Six hundred eighteen women undergoing ovarian stimulation with or without IUI treatment between November 2001 and January 2003. Of these, 254 were self-identified as lesbians and 364 were heterosexual women. Baseline pelvic ultrasound examination and blood tests conducted to measure biochemical parameters such as FSH , LH, E2, PRL, T, androstenedione (A), sex hormonebinding globulin (SHBG), and DHEAS were performed between day 2 and 3 of each woman’s menstrual cycle. Tubal patency tests were performed by hysterosalpingography or laparoscopy. Eighty percent of lesbian women, compared with 32% of the heterosexual women, had PCO on pelvic ultrasound examinatio n. Thirtyeight percent of lesbian women, compared with 14% of heterosexual women, had PCOS. There were no significant differences in the androgen concentrations between lesbians and heterosexual women with normal ovaries. However, lesbian women with PCO and PCOS had significantly higher androgen concentrations were compared with heterosexual women with PCO and PCOS. Tubal disease was as common in lesbian women as in heterosexual women. There is a pronounced higher prevalence of PCO and PCOS in lesbian compared with heterosexual women. Lesbian women with either PCO or PCOS had more pronounced hyperandrogenism than did heterosexual women with either PCO or PCOS.