A group of 992 breast cancer patients (risk group, R) was compared with a group of 482 patients hospitalized for non oncologic reasons and matched for age and year of hospitalization (comparison group, C). The findings confirm the following factors as risk factors for breast cancer: nulliparity (R 28.8%, C 17.5%, p less than 0.001), late first birth (over 34 years of age) (R 11.4%, C 5.1%, p less than 0.001), diabetes mellitus (R 7.0%, C 3.8%, p = 0.017), hypertension (R 25.7%, C 18.1%, p = 0.0016), alcohol (R 9.4%, C 5.9%, p = 0.03), positive family history (R 14.8%, V 5.3%, p less than 0.001) and breast surgery for benign disease (R 13%, C 7.5%, p = 0.002). Frequently mentioned risk factors such as early menarche and late menopause did not emerge as risk factors in our study. Cigarette smoking did not show a protective effect but even tended to be more frequent in the risk group. Multiparity (more than 2 births) was protective (R 22.1%, C 32.4%, p less than 0.001). The findings on hormonal replacement therapy (R 7.1%, V 17.0%, p less than 0.01) might have been influenced by a selection bias (hospitalization of patients in the comparison group because of complications of hormonal replacement therapy such as bleeding) and are thus not fully conclusive. It can at least be said that hormonal replacement therapy is not more frequent in the risk group.