Parity has been studied extensively as a risk factor for colorectal cancer but has not been definitively shown to be associated with altered risk. In a few studies, risk of colorectal cancer in childless men has been compared to risk in men with children, but results have not been consistent. We analyzed the association of fatherhood with risk of colorectal adenomas in male self-defense officials (ages 49-55) in Japan. The study participants received a preretirement health examination including flexible sigmoidoscopy at Self-Defense Forces hospitals in Japan from January 1991 through December 1992. The examinations identified 265 cases with rectal or sigmoid adenomas and 1480 controls with normal examinations up to 60 cm from the anus. Data on marital status, number of children, long-term work assignment away from wife and children, and other lifestyle variables were obtained by means of a self-administered questionnaire prior to physical examination. Multiple logistic regression analysis assessed the risk of adenomas in relation to number of children, marital status, long-term work assignment away from family, and military rank, with adjustment for cigarette smoking, alcohol intake, dietary variables, body mass index, and recreational physical activity. In this relatively homogeneous group, more than 98% of both cases and controls were currently married, and more than 93% had children. The adjusted odds ratio for the association of adenomas with fatherhood was 0.4 (95% confidence interval, 0.2-0.8). Marital status and work assignment away from the family were not associated with adenoma risk. These findings suggest that colorectal adenomas and perhaps cancer risk may be associated with childlessness in men.