Recent data in the mouse demonstrate that leptin, a protein hormone produced by fat cells, is required for fertility. In the absence of leptin the mice become obese, diabetic and infertile. Polycystic ovary syndrome (PCOS), a common cause of infertility in women, is associated with obesity and insulin resistance. Because of the increased frequency of PCOS in obese women we tested the hypothesis that alterations in serum leptin concentrations might be associated with PCOS. Immunoreactive leptin concentrations were measured in 58 women with PCOS and 70 regularly menstruating (control) women. As has previously been shown there was a positive correlation between leptin levels and body mass index (BMI). Although the leptin levels in the majority of women with PCOS fell within the control range, 29% of PCOS women had leptin levels above the 99% prediction interval for their BMI and none had low leptin levels. There were also positive correlations of leptin levels with free testosterone and insulin sensitivity in control women. In women with PCOS, 13% and 9.5% exhibited higher than expected leptin concentrations with respect to free testosterone and insulin sensitivity, respectively. Insulin resistant PCOS women had higher leptin levels than controls. The data demonstrate that a substantial proportion of women with PCOS have leptin levels that are higher than expected for their BMI, free testosterone and insulin sensitivity. These results suggest that abnormalities in leptin signaling to the reproductive system may be involved in certain cases of PCOS.