Adiponectin is an adipocyte-derived protein with an insulin-sensitizing action. Circulating levels of adiponectin are inversely correlated with obesity, especially abdominal obesity. Some studies have suggested that low levels of circulating adiponectin might be related to increased risk of colorectal cancer and adenomas. The present study examined the relationship between total and high-molecular-weight (HMW) adiponectin to colorectal adenomas in the Self Defense Forces (SDF) Health Study. The study subjects comprised 656 cases of colorectal adenomas and 648 controls with normal colonoscopy among men receiving a preretirement health examination at two Self Defense Forces hospitals. Total and HMW adiponectin were slightly lower in adenoma cases than in controls; geometric means of total adiponectin were 5.42 microg/mL in cases and 5.63 microg/mL in controls (P = 0.13), and the corresponding values of HMW adiponectin were 2.47 microg/mL and 2.57 microg/mL, respectively (P = 0.29). Regardless of adjustment for body mass index and other lifestyle factors, total adiponectin was unrelated to the risk of colorectal adenomas. Total adiponectin levels were inversely related to the risk of large adenomas (>or= 5 mm), but not of small adenomas, with a nearly statistically significant decreasing trend (P = 0.06). However, the inverse association was largely ascribed to body mass index and other lifestyle factors. HMW adiponectin showed no clear association with either overall or size-specific risk of colorectal adenomas. The study provided suggestive evidence for a protective association between adiponectin and large adenomas, but did not indicate a protective association independent of adiposity.