Among 118,404 female registered nurses who responded to a mailed questionnaire and had no history of cancer in 1976, 191 colon cancers and 49 rectal cancers were diagnosed during 8 years of follow-up. We observed no material association between parity and colon cancer risk; women who bore four or more children had a risk similar to that of nulliparous women (age-adjusted RR = 1.1, 95% confidence limits (CL): 0.6, 1.9). No consistent patterns of colorectal cancer risk were associated with age at menarche or at first birth. Current or past use of oral contraceptives did not appreciably alter the risk of colorectal cancer; past use of postmenopausal estrogens was associated with a reduced risk of colorectal cancer (RR = 0.5, 95% CL: 0.3, 1.0). These and other data do not support any substantial effect of reproductive history or oral contraceptive use on the risk of large bowel cancer; however, the possible protective role of postmenopausal estrogen therapy merits further investigation.