[en] A case-control study was made on female breast cancer cases and their matched controls in the Life Span Study sample. The index cases were detected during 1958-69 among the 251 breast cancer cases ascertained originally by McGregor et al. The purpose of this study was to define the epidemiologic risk factors of breast cancer among Japanese women, to test for radiation effects in the presence of other risk factors, and to search for interactions. The survey was conducted by interview at home visits for those residing in the Hiroshima and Nagasaki areas, and by mail survey for others. The interview was carried out by several trained interviewers. Information concerning suspected risk factors of breast cancer, such as familial history, education, age at menarche and menopause, marital history, reproductive history, history of breast feeding, etc., was collected for both index cases and controls. Out of 183 original pairs, analysis was made on 164 pairs with available information for both the index and control, using the method of matched samples described by Mantel and Haenszel. There was enhancement of risk for those exposed to high radiation dose (100 rad or more). Although most major results were similar to those of previous studies, a significant increase of risk was observed among those under one of the following conditions: actual duration of marriage was less than 10 years; number of pregnancies was two or less; and age at delivery of first live born child was 27 or over. These factors had a mutual interrelationship and cases with two or more of these risk factors showed higher risk than those with one. Additive interrelationship was demonstrated between radiation dose and these marital or reproductive risk factors in elevating the relative risk of breast cancer. (author)