We evaluated the effect of smoking on the risk of developing colorectal adenoma in men. The data were obtained in a case-control study of 103 men with colorectal biopsy-proven adenoma (cases) and 108 men with normal colonoscopy findings (controls). As compared with men who had never smoked, the estimated relative risk of adenoma increased with the pack-year smoking number, the average number of cigarettes per day, and the total years smoked. The estimated overall relative risk was 2.2 (95% confidence interval, 1.1 to 4.3). Adjustment for age did not alter these results. The data suggest that smokers may have a higher risk of developing colorectal adenoma than non-smokers. If these results are confirmed, they might be of direct importance for public health, since adenomas are precursors of colorectal cancer.