Circulating levels of adiponectin, a hormone with insulin-sensitizing properties, are decreased in conditions related to obesity and hyperinsulinemia, which are recognized risk factors for endometrial cancer. Eighty-seven cases with incident, histologically confirmed endometrial cancer and 132 controls admitted for acute, nonneoplastic diseases were interviewed in northeastern Italy between 1999 and 2002, and blood samples were collected. Levels of adiponectin were evaluated in samples by means of a RIA. An inverse association with endometrial cancer risk emerged for plasma adiponectin levels [odds ratio (OR), 0.42; 95% confidence interval, 0.19-0.94] when comparing the highest vs. the lowest tertiles. Similar results emerged for serum adiponectin (OR, 0.30; 95% confidence interval, 0.14-0.68). The association was stronger in pre- than in postmenopausal women, but no significant heterogeneity was observed across strata of body mass index (BMI) or parity. BMI and adiponectin showed independent effects on the risk of endometrial cancer according to a multiplicative model (OR, 6.45 in the highest level of BMI and in the lowest one of adiponectin).