We examined the relationship between body mass [weight (kg)/height (m)2] and breast cancer using data from the Cancer and Steroid Hormone Study. The study compared 4323 women aged 20-54 years with newly diagnosed breast cancer identified through population-based tumor registries with 4358 women randomly selected from the general population of the same geographic areas. Among naturally menopausal women, risk of breast cancer increased with increasing body mass index (BMI); those severely overweight (BMI greater than or equal to 32.30) had nearly 3-fold higher risk of breast cancer compared with women in the leanest category (BMI less than 20.00). This positive association appeared stronger with increasing years since menopause and in women who had ever used estrogen replacement therapy. A positive association between body mass and breast cancer risk also was observed among premenopausal women; however, risk estimates were substantially lower. Substantial weight gain from adolescence to adulthood was a more important risk factor than lifelong obesity. Prevalence of obesity increases with age; our results suggest that interventions that prevent this trend could have an important effect on breast cancer risk, especially during the menopausal years.