Selenium is an essential trace element that has been implicated in cancer risk; however, study results have been inconsistent with regard to colon cancer. Our objectives were to 1) investigate the association between selenium and colon cancer, 2) evaluate possible effect measure modifiers, and 3) evaluate potential biases associated with the use of postdiagnostic serum selenium measures. The North Carolina Colon Cancer Study is a large population-based, case-control study of colon cancer in North Carolina between 1996 and 2000 (n = 1,691). Nurses interviewed patients about diet and lifestyle and drew blood specimens, which were used to measure serum selenium. Individuals who had both high serum selenium (> 140 mcg/l) and high reported folate (> 354 mcg/day) had a reduced relative risk of colon cancer [odds ratio (OR) = 0.5, 95% confidence interval (CI) = 0.4-0.8). The risk of colon cancer for those with high selenium and low folate was approximately equal to the risk among those with low selenium and low folate (OR = 1.1, 95% CI = 0.7-1.5) as was the risk for those with low selenium and high folate (OR = 0.9, 95% CI = 0.7-1.2). We did not find evidence of bias due to weight loss, stage at diagnosis, or time from diagnosis to selenium measurement. High levels of serum selenium and reported folate jointly were associated with a substantially reduced risk of colon cancer. Folate status should be taken into account when evaluating the relation between selenium and colon cancer in future studies. Importantly, weight loss, stage at diagnosis, or time from diagnosis to blood draw did not appear to produce strong bias in our study.