Register and census data for complete cohorts of Norwegian men and women born between 1935 and 1969 have been used to examine the relationship between reproductive factors and the incidence of colorectal cancer. Among 1.1 million men and 1.1 million women under observation, 491 male and 859 female cases of colorectal cancer were diagnosed during the period of follow-up. Our hazard model estimates clearly show that in these young cohorts, women with 2 or more children run a lower risk of having a malignant tumor in the cecum or ascending colon than do other women of the same age and in the same birth cohort. No association with parity is found with respect to cancer in the transverse or descending colon, whereas a downward trend in the effect estimates, followed by an upturn, appears for the rectum, sigmoid colon and rectosigmoid junction. Such correlations, which also hold when we control for education and place of residence, are not found for men. The observed relationship between parity and cancer incidence is not likely to be exclusively explained by life-style differentials. Presumably, there is a biological effect of the number of pregnancies and deliveries, net of age at first birth and other reproductive factors, on the development of colorectal cancer.