Reproductive and menstrual characteristics were ascertained by telephone interview from Wisconsin women with newly reported diagnoses of carcinoma of the colon (n = 536) and rectum (n = 243) and 2315 population control subjects. The relationships of parity, age at first full-term pregnancy, age at menarche, regularity and length of menstrual cycles, age at menopause, and type of menopause to colon and rectal cancer were examined. In our study population, no reproductive or menstrual characteristics were significantly associated with colon cancer, although subsite analyses suggested an increased risk for transverse colon cancer in women reporting irregular menstrual cycles. Only parity was associated with rectal cancer; women who had given birth five or more times were at significantly lower risk compared to those who were nulliparous (odds ratio: 0.60; confidence interval: 0.36, 0.99). The odds ratio per birth also suggested an inverse association. These data give little support to the hypothesis that female reproductive events modify colon and rectal cancer risk.