Between 1982 and 1985, a case-control study of nulliparous women, aged 25-45, was conducted to analyse the relationships between the risk of breast cancer and causes of nulliparity, including contraceptive methods. Fifty-one cases of breast cancer diagnosed less than 3 months before interview were matched with 95 controls on age at diagnosis, year of interview, and medical center. The causes of nulliparity related to female sterility or subfertility (mechanical or hormonal disorders) were not found to be associated with a significantly higher risk of breast cancer. The causes related to fertilization failure, i.e. no sexual partner, rare sexual intercourse (less than once per month), or partner with abnormal semen, were found to lead to an increased risk. Detailed analysis of contraceptive methods showed that the risk of breast cancer increased (p = 0.02) with a longer duration of use of barrier methods (withdrawal or condom). Conversely, the risk significantly decreased (p = 0.004) with a longer duration of use of non-barrier methods (oral contraceptives, IUD, cap, local spermicides, vaginal douche, safe period, or no method), i.e. methods allowing a direct exposure to human semen.