The authors prospectively investigated the relation between stressful life events and risk of breast cancer among 10,808 women from the Finnish Twin Cohort. Life events and breast cancer risk factors were assessed by self-administered questionnaire in 1981. A national modification of a standardized life event inventory was used, examining accumulation of life events and individual life events and placing emphasis on the 5 years preceding completion of the questionnaire. Through record linkage with the Finnish Cancer Registry, 180 incident cases of breast cancer were identified in the cohort between 1982 and 1996. The multivariable adjusted hazard ratio for breast cancer per one-event increase in the total number of life events was 1.07 (95% confidence interval (CI): 1.00, 1.15). This risk estimate rose to 1.35 (95% CI: 1.09, 1.67) when only major life events were taken into account. Independently of total life events, divorce/separation (hazard ratio (HR) = 2.26, 95% CI: 1.25, 4.07), death of a husband (HR = 2.00, 95% CI: 1.03, 3.88), and death of a close relative or friend (HR = 1.36, 95% CI: 1.00, 1.86) were all associated with increased risk of breast cancer. The findings suggest a role for life events in breast cancer etiology through hormonal or other mechanisms.