BACKGROUND: Cohort studies investigating the association between stress and breast cancer have shown highly inconsistent results. METHODS: The Women's Lifestyle and Health Cohort Study included 36,332 Swedish women age 30-50 years who were employed at baseline (1991-1992). Participants were followed through December 2004 using linkages to national registries. A total of 767 women were diagnosed with breast cancer during follow-up. RESULTS: Among women working full-time, low job control and high job demands were weakly associated with breast cancer risk (hazard ratios of 1.2; 95% CI = 1.0-1.5). Women with both low job control and high job demands ("job strain") had higher risk of breast cancer than women with high job control and low demands ("low strain") (1.2; 0.9-1.6). Multivariate adjustment slightly strengthened the association between breast cancer and job strain (1.4; 1.1-1.9), whereas the associations with control and demands were unchanged. Work characteristics were unrelated to breast cancer risk among women working part-time. DISCUSSION: There was a small increased risk of breast cancer among women in full-time employment who experienced job strain, but not among part-time workers.