This study aimed to assess the familial relative risk for colorectal cancer (CRC) and its variation according to age and gender. A population-based family study was carried out in France, from 1993 to 1998, including 761 families. Familial CRC risks were estimated from a cohort analysis of the relatives. No obvious decrease in CRC risk was found with increasing age, except when either the proband, or the relative, were in the youngest age class. The effect of the relatives' and probands' ages on the CRC risk differed according to their gender. The cumulative risk of CRC increased at an earlier age in male relatives of probands younger than 60 years of age, than in female relatives. This result suggests that mechanisms specific to females, possibly interacting with genetic factors, explain the difference in the cumulative risks between families with male and female probands.