Historically, breast cancer rates have been low in Asia but rates have increased substantially in Asian-Americans for reasons that are not well understood. The authors conducted a population-based case-control study of breast cancer in Los Angeles County, which included 1,277 (450 Chinese, 352 Japanese, 475 Filipinos) women with incident, histologically confirmed breast cancer and 1,160 control subjects (486 Chinese, 311 Japanese, 363 Filipinos). A detailed in-person interview was conducted, which included questions on menopausal hormone therapy (HT) use, height, weight in each decade of life and reproductive factors. Breast cancer risk increased with increasing recent weight in postmenopausal women (p trend = 0.015). There was a significant 16% (95% CI = 2-35%) increase in risk per 10 kg of body weight in postmenopausal women. In both premenopausal and postmenopausal women, risk increased with increasing waist to hip ratio; this remained statistically significant after adjustment for recent weight in all subjects combined (p trend = 0.042). The increased risk associated with high recent weight in postmenopausal women was more apparent for women with high waist to hip ratio (p trend = 0.013). Use of HT was a significant risk factor; risk increased 26% per 5 years of current use of estrogen and progestin therapy (p trend = 0.017). The increased risk associated with high body weight was observed irrespective of HT use. Use of HT and high body size might have contributed to the rapid increase of breast cancer in Asian-Americans.