BACKGROUND: Data from the Carolina Breast Cancer Study, a population-based, case-control study of breast cancer in African-American and white women residents of North Carolina, were evaluated to determine whether specific aspects of lactation are associated with a reduction in the risk of breast cancer. METHODS: Analyses included 751 parous cases and 742 parous controls frequency-matched on age and race. Information on lactation, reproductive history, lifestyle characteristics and family history were obtained through a personal interview. RESULTS: When women who breastfed were compared to those who never breastfed, odds ratios and 95% confidence intervals of 0.8 (0.5-1.1) and 0.7 (0.5-0.9) were found for women 20-49 years and 50-74 years, respectively. Similar inverse associations were observed for each of three categories of lifetime duration (1-3, 4-12, 13+ months). The inverse associations persisted and did not vary when number of children breastfed, ages at first and last lactation and lactational amenorrhoea were examined. CONCLUSIONS: Our findings suggest that any lactation, regardless of duration or timing, is associated with a slight reduction in the risk of breast cancer among younger and older parous women.