Abstract Obesity has been associated with an increased risk of colorectal neoplasia, but the mechanisms of this potential association have not been elucidated. We hypothesized that the adipokines adiponectin, leptin, and tumor necrosis factor-α (TNF-α) may mediate an association between obesity and colorectal cancer. We measured plasma concentrations of total and high-molecular-weight (HMW) adiponectin, leptin, and TNF-α in healthy volunteer examinees who underwent total colonoscopy between February 2004 and February 2005, and conducted a case-control study consisting of 778 cases and 735 controls. An inverse association of total and HMW adiponectin was observed with colorectal adenoma (P trend < 0.001 and 0.03, respectively). Further, total adiponectin interacted with leptin, but not TNF-α, in relation to colorectal adenoma (P interaction = 0.007). An inverse association of total adiponectin with colorectal adenoma was apparent in the highest two tertiles of leptin, particularly the middle (P trend < 0.001), whereas a positive association of leptin was obvious in the lowest tertile of total adiponectin (P trend = 0.01) after adjusting for potential confounders and body mass index, which is a major determinant of insulin resistance. Adiponectin may exert an anticarcinogenic effect on the large intestine by interfering with leptin, whereas leptin could conversely exert a carcinogenic effect under conditions of a lower abundance of adiponectin. Our findings provide the first epidemiologic evidence for interactive effects of adiponectin and leptin in the early stage of colorectal tumorigenesis, distinct from their involvement in insulin resistance. Cancer Res; 70(13); 5430–7. ©2010 AACR.