Studies have shown fairly consistent results regarding an inverse relationship between overweight and premenopausal breast cancer risk, but reported effects of weight changes have been inconsistent. Data was analysed on weight, weight changes, height and body build for 558 premenopausal women with breast cancer and 1116 controls below 51 years who participated in a population-based case-control study in Germany from 1992 to 1995. Larger body build at menarche conferred a protective effect, when compared with smaller build [odds ratio (OR) 0.69, 95% confidence interval (CI) 0.49, 0.96]. In comparison with the lowest quartile, higher quartiles of weight gain from the lowest adult weight to current weight conferred a significant protective effect to those who attained their lowest adult weight after but not before the (median) age of 21, with OR and 95% CI of 0.52 (0.32, 0.83), 0.50 (0.30, 0.81), 0.56 (0.34, 0.94) for second, third and fourth quartiles respectively. Our findings are consistent with studies that found weight gain to be protective against premenopausal breast cancer and suggest that this effect may be more pronounced for women who were lean in adolescence and early adulthood.