BACKGROUND Breast cancer incidence rates have historically been four to seven times higher in the United States than in China or Japan, although the reasons remain elusive. When Chinese, Japanese, or Filipino women migrate to the United States, their breast cancer risk rises over several generations and reaches that for white women in the United States, indicating that modifiable exposures are involved. In a previous report on this case-control study of breast cancer in Asian-American women, designed to take advantage of their diversity in risk and lifestyle, we demonstrated a sixfold gradient in risk by migration history, comparable to the international differences in breast cancer incidence rates. OBJECTIVE In this analysis, we have examined the roles of adult height, adiposity, and weight change in breast cancer etiology. METHODS A population-based, case-control study of breast cancer was conducted among women of Chinese, Japanese, and Filipino ethnicities, aged 20-55 years, living in San Francisco-Oakland (CA), Los Angeles (CA), and Oahu (HI) during the period from April 1, 1983, through June 30, 1987. We successfully interviewed 597 (70%) of 852 eligible case subjects and 966 (75%) of 1287 eligible control subjects from August 1985 through February 1989. Subjects were asked about current height, usual adult weight, and usual weight in each decade of life, excluding the most recent 3 years and any periods of pregnancy. RESULTS Height, recent adiposity (weight in the current decade of life/height 1.5), and recent weight change (between the current and preceding decades of life) were strong predictors of breast cancer risk after adjustment was made for accepted breast cancer risk factors. Risk doubled (relative risk [RR] = 2.01; 95% confidence interval [CI] = 1.16-3.49) over the 7-inch (17.8-cm) range in height (two-sided P for trend = .003), with comparable effects in both premenopausal and postmenopausal women. Except for reduced risk in the heavy, younger women (weight/height 1.5 > 29 kg/m 1.5 and < 40 years old), risk was positively associated with usual adult adiposity. Trends in risk became more striking as adiposity in each succeeding decade of adult life was considered. Women in their 50s and in the top quintile for their age group had twice the breast cancer risk (RR = 2.13; 95% CI = 1.17-3.87) of women in the bottom quintile (two-sided P for trend = .004). Women in their 50s, above the median adiposity for their age group, and with a recent gain of more than 10 pounds had three times the risk (RR = 3.01; 95% CI = 1.45-6.25) of women below the median adiposity and with no recent weight change. Recent weight loss was consistently associated with reduced risk (RRs of approximately 0.7) relative to no recent weight change. CONCLUSIONS Adult adiposity, weight change, and height are critical determinants of breast cancer risk. Increased adiposity and weight gain in the decade preceding diagnosis are especially influential, suggesting that excess weight may function as a late stage promoter. CONCLUSIONS Weight maintenance and/or reduction as an adult, possibly accompanied by specific changes in diet and physical activity, may have a significant and rapid impact on breast cancer risk.