A case-control study was carried out in Spain to assess associations between parity, lactation and age at first full-term pregnancy and breast cancer. From November 1989 to February 1992, 184 incident breast cancer histologically confirmed cases were interviewed and matched by age and residence to 184 hospitalized patients and 184 community controls selected by random digit dialing. Multiple logistic regression was used to assess the independent influence of each factor on the risk of breast cancer in relation to other factors included in the model. Age at first full-term pregnancy was associated with breast cancer risk with an estimated odds ratio of 3.5 (95% CI 1.41-9.83) for women with their first birth after 30 years in comparison with those whose first birth was before age 21. Breast cancer risk decreased with increasing number of full-term pregnancies, OR 0.3 (95% CI 0.16-0.78) for women who had had more than 3 full-term pregnancies in comparison with nulliparous women. Among parous women, the estimated OR for women with more than 3 children was 0.4 (95% CI 0.13-0.81) after allowance for age at first childbirth and lactation. The estimated OR was 2.6 (95% CI 1.4-4.7) for women with a positive history of breast cancer in first-degree relatives. Breast cancer was not associated with total duration of lactation. The study indicates that parity is an independent risk factor associated to breast cancer and that the women with a late age at first full-term pregnancy constitute a high-risk group.