Evidence for an association between reproductive factors, exogenous female hormone use, and colorectal cancer risk from previous epidemiological studies remains controversial and information from nonwestern populations is limited. We analyzed this association in the Japan Public Health Center-based Prospective Study, conducted in 48 511 Japanese women aged 40-69 years who responded to a self-administered questionnaire that included history of reproductive factors, exogenous female hormone use, and other factors. During a mean follow-up of 12 years, a total of 538 colorectal cancer cases were newly identified. Age at menarche, menopausal status, history of exogenous female hormone use, parity, number of births, age at first birth, history of breast feeding, and reproductive period (postmenopausal women only) were not associated with colorectal cancer. When colon and rectal cancer were, however, analyzed separately among all women, multivariate-adjusted hazard ratios (95% confidence intervals) for colon cancer for age at first birth of 23-25, 26-29, 30 years or more in comparison to that at 22 years or less were 0.84 (0.64-1.12), 0.73 (0.53-1.01), and 0.66 (0.41-1.09), respectively (P for trend=0.03). These results were strengthened for colon cancer when analysis was restricted to postmenopausal women (P for trend=0.01), but no association was seen in premenopausal women (P for trend=0.59). In contrast, no statistically significant association was found for rectal cancer. These findings suggest that late age at first birth is associated with a reduced risk of colon cancer in postmenopausal Japanese women.