Abstract Objective: To investigate the strength of association between past life events and the development of breast cancer. Design: Case-control study. A standardised life events interview and rating was administered before a definitive diagnosis. Setting: Breast Cancer Screening Assessment Unit and surgical outpatient clinics at King's College Hospital, London. Subjects: 119 consecutive women aged 20-70 who were referred for biopsy of a suspicious breast lesion. Main outcome measures: Odds ratio of the risk of developing breast cancer after life events in the preceding five years after adjustment for confounders. Results: 41 women were diagnosed as having malignant disease while the remainder had benign conditions. Severe life events increased the risk of breast cancer. The crude odds ratio was 3.2 (95% confidence interval 1.35 to 7.6). After adjustment for age and the menopause and other potential confounders this rose to 11.6 (3.1 to 43.7). Multiple logistic regression analysis showed that all severe events and coping with the stress of adverse events by confronting them and focusing on the problems significantly predicted a diagnosis of breast cancer. Non-severe life events and long term difficulties had no significant association. Conclusion: These findings suggest an aetiological association between life stress and breast cancer.