To explore the possible explanation of the marked difference in the incidence of breast cancer between Chinese and Australian women, the authors have compared and analysed the results of two case-control studies completed recently in Tianjin, China and Adelaide, Australia. Of 10 known risk factors for breast cancer 9 were significantly higher in level in Adelaide women than in Tianjin women, i.e., women in Adelaide were much taller, heavier, more obese, earlier at menarche, later at first full-term pregnancy, more nulliparous, less parity, more in history of breast cancer in first degree relatives, more educated. From the 7 risk factors analysed by use of logistic regression model 5 factors, i.e., earlier age at menarche, later age at first full-term pregnancy, less parity, history of benign disease, and history of breast cancer in first degree relatives, were associated significantly with the increased risk for breast cancer in the Tianjin study, but not in Adelaide study. These factors being not assoc