Our aim was to determine whether serum leptin level is regulated by thyroid hormones, lipid metabolic products and insulin resistance status in women with polycystic ovary syndrome (PCOS). A prospective case-controlled study was carried out in Istanbul University, Cerrahpasa School of Medicine in 25 lean PCOS (L-PCOS) women, 19 obese PCOS (O-PCOS) women and 28 normal women. The diagnosis of PCOS was established according to the clinical, hormonal (elevated luteinizing hormone and serum androgens) and ultrasonographic findings. Fasting serum levels of thyroid stimulating hormone (TSH), free triiodothyronine (FT3), free thyroxine (FT4), fasting glucose, insulin, total cholesterol (TC), triglyceride (TG), high-density lipoprotein-cholesterol (HDL-C), low-density lipoprotein-cholesterol (LDL-C), very low-density lipoprotein-cholesterol (VLDL-C) and leptin were measured and compared in the three groups and the correlations between serum levels of leptin and other parameters were evaluated. Serum leptin levels were higher in the O-PCOS group, while its level was comparable between the L-PCOS and control groups. Serum levels of FT4 were significantly lower in both L-PCOS and O-PCOS groups than the control group. Women in both L-PCOS and O-PCOS groups were found to be significantly hyperinsulinemic and insulin resistant. Serum levels of TC, VLDL-C and TG were significantly higher in the O-PCOS group, while serum HDL-C level was lower. There was a poor correlation between serum leptin, and FT4, TC, TG, HDL-C and VLDL-C levels. A significant correlation was observed between serum leptin levels and both BMI and insulin resistance status in PCOS. We believe that, although thyroid hormones and lipid metabolic products do not seem to participate in the regulation of serum leptin levels, BMI and insulin resistance status may have a key role in women with PCOS.