The polycystic ovary syndrome (PCOS) is characterized by men- strual disturbances, chronic anovulation and hyperandrogenism and is associated with insulin resistance and hyperinsulinemia. Leptin, the product of the ob gene, is an adipocyte-secreted molecule that signals the magnitude of energy stores to the brain and has been recently shown to have important effects on the reproductive axis of rodents. To assess the potential contribution of leptin to the patho- genesis of PCOS, we measured leptin levels in 24 obese women with PCOS and 12 weight- and age-matched controls and determined whether alterations in hyperinsulinemia produced by administration of the insulin-sensitizing agent troglitazone had an effect on serum leptin levels. Leptin concentrations at baseline were not different in women with PCOS (38.1 6 2.15 ng/mL) and controls (33.12 6 2.39 ng/mL). Moreover, leptin concentrations remained unchanged after treatment with troglitazone (38.1 6 2.15 vs. 39.21 6 2.65 ng/mL). Baseline leptin correlated strongly with body mass index in both controls (r 5 0.59; P , 0.05) and women with PCOS (r 5 0.70; P 5 0.0004). Leptin levels were not associated with baseline insulin, tes- tosterone, non-sex hormone-binding globulin (SHBG)-bound testos- terone, dehydroepiandrosterone sulfate, estradiol, or SHBG. Finally, despite significantly reduced insulin, non-SHBG-bound testosterone, and estradiol levels after troglitazone treatment of women with PCOS, their leptin levels remained unchanged. We conclude that circulating leptin levels in patients with PCOS do not differ from those in age- and weight-matched controls. Furthermore, increased circu- lating insulin due to insulin resistance does not appear to alter cir- culating leptin levels in women with PCOS. (J Clin Endocrinol Metab 82: 1687-1691, 1997)