Obesity is associated with an increased risk of colorectal cancer. Circulating levels of leptin are high in obesity and strongly correlated to levels of insulin. Leptin stimulates growth of colon cancer cells. In a nested case-control study, we measured leptin levels in prediagnostic plasma from 75 men and 93 women who were diagnosed with colorectal cancer mean time 3.4 years (SD 2.4) after recruitment and among 327 control subjects. Logistic regression analyses showed increases in colorectal cancer risk in men with increasing levels of leptin, odds ratios (OR) were 1.00 (ref), 0.85 (95% C.I.=0.33-2.23), 1.04 (0.43-2.53), and 2.15 (0.89-5.22), (pfor trend=0.08). There was a distinct threshold between the third and fourth quartile of leptin, and the odds ratio for top quartile vs. three bottom quartiles was 2.28 (1.09-4.76). Adjustment for body mass index and insulin did not affect risk estimates. In separate analysis, odds ratio for top vs. bottom tertile of colon cancer was 1.96 (95% C.I.=0.72-5.29), whereas no increase was seen for rectal cancer. In women, no association between leptin and risk was seen. These data support the hypothesis that leptin is a risk marker for colorectal cancer in men, but not in women.