A comparison of epidemiological characteristics in breast cancer patients and normal women in Great Britain and Japan: results of a prospective study.

The epidemiological characteristics of normal women and women with early breast cancer in Great Britain were prospectively compared with similar subjects from Japan. The study consisted of 204 women with early breast cancer and 792 normal controls from Great Britain and 200 breast cancers and 335 Japanese controls. The risk factors examined were age, height and weight, menopausal status, age at menarche and menopause, parity, age at first birth, use of oral contraceptives, and family history of breast cancer. The cancers and cases were divided into pre- and postmenopausal groups. There was a striking trend for a protective effect of multiparity in both countries and a weak but inconstant protective effect of age at first birth. In premenopausal British women increase in body mass was significantly associated with a decreased risk of breast cancer but this association was not found for postmenopausal women or in the Japanese women. The results showed that the classical risk factors did not fully account for the differences in breast cancer risk between Great Britain and Japan. The possibility that aetiological factors might be changing warrants investigation.