The belief that life stress enhances breast cancer is common, but there are few prospective epidemiological studies on the relationship of life stress and breast cancer. We have investigated the association between stress of daily activities (SDA) and breast-cancer risk in a prospective cohort study of 10,519 Finnish women aged 18 years or more. SDA measures a subject's own appraisal of daily stress. It was assessed in 1975 and 1981 by a self-administered questionnaire, which also provided information on subject characteristics and other known breast-cancer risk factors. Follow-up data for breast cancer from 1976 to 1996 were attained through record linkage to the Finnish Cancer Registry. Study subjects were divided into 3 groups based on their SDA scores in 1975: no stress (23% of subjects), some stress (68%) and severe stress (9%). Hazard ratios (HRs) and respective 95% confidence intervals (CIs) for incidence of breast cancer by level of SDA were obtained from the Cox proportional hazards model. We identified 205 incident breast cancers in the cohort. Multivariable-adjusted HRs for breast-cancer risk were 1.00 (reference), 1.11 (95% CI 0.78-1.57) and 0.96 (95% CI 0.53-1.73) by increasing level of stress. Neither shifting of the SDA cut-off points nor restricting the analysis to women who reported the same level of SDA in 1975 and 1981 materially altered the results. We found no evidence of an association between self-perceived daily stress and breast-cancer risk.