In a case-control study of 495 breast cancer patients and 785 controls between 20 and 56 years of age, the risk of breast cancer associated with a family history of breast cancer was studied according to age and reproductive factors. The familial risk of breast cancer was not significantly modified by age at onset, age at menarche, number of children, age at first full-term pregnancy, menstrual cycle length or age at menopause. However, the familial risk significantly increased with the number of abortions (p < 0.05) and seemed to decrease after a natural menopause (p = 0.08). These results suggest that a familial predisposition to breast cancer exerts the same influence during the first six decades of life, except maybe when there are isolated or repeated events such as abortions or artificially imposed menopause, in which case the risk is apparently greater.