PURPOSE: Because the breast cancer is one of the major causes of mortality among women, it is important to identify modifiable risk factors for this disease. Some reported that overweight/obesity is a risk factor for breast cancer, but the results are not consistent. Human breast cancer has different characteristics according to the status of menopause (premenopause and postmenopause). For the premenopaused women, the majority of the relevant prospective studies support an inverse relationship between body mass index and the development of breast cancer. In contrast, for the postmenopaused women, a positive relationship between body mass index and development of breast cancer has been reported in only half of prospective studies on this topic. This study was undertaken to determine the role of body size and body mass index by status of menopause in development of breast cancer in Korea using retrospective case-control study. MATERIALS AND METHODS: The breast cancer cases (n=683) and controls (n=501) were recruited from January 1993 to April 1998 at the Asan Medical Center. The authors collected information on demographic, reproductive and anthropometric characteristics by interviews. Quetelets index was calculated using height and weight. Multiple logistic regression was done to estimate adjusted odds ratios (ORs) by menopausal status, controlling age, age at first full-tenn pregnancy, age at menarche, number of parity, family history of breast cancer. RESULTS: Overall, there was a moderate, but significant association between obesity and breast cancer. Among premenopausal women, in comparison with women whose weights were less than 50 kg, the ORs for the upper quintile group of weight was 1.71 (95% confidence interval (CI), 1.01~2.89). The heaviest premenopausal women had a higher risk (OR=1.16, 95% CI, 1.05~1.29, P=0.005). The adjusted OR for the highest quintile of BMI relative to the lowest was 2.02 (95% CI, 1.18~3.45). Higher body mass index was significantly associated with increased risk of premenopausal breast cancer (OR=1.08, 95% CI, 1.02~1.15, P=0.006). Among postmenopausal women, higher body mass index and weight were associated with increased risk of breast cancer. In comparison with women whose weights were less than 50 kg, the OR for the upper quintile group of weight was 2.08 (95% CI, 1.064.08). The adjusted OR for the highest quintile of BMI relative to the lowest was 2.02 (95% CI, 1.02~4.01). CONCLUSION: Our findings suggest that overweight/obesity may play an important role in the incidence of premenopausal and postmenopausal breast cancer in Korea.