A study of 1529 breast cancer patients and 1901 control subjects enabled evaluation of risk in relation to lifetime changes in body size. Tallness, regardless of when achieved, was associated with an increased risk of breast cancer diagnosed at both young (< 50 years) and older ages, with adult height of 68 in or more increasing risk by nearly 50 to 80% compared with statures of less than 62 in. The association of risk with weight was less clear. Subjects who described themselves as heavier than average at ages 8 to 9 or 16 years were at reduced risk, particularly for older-onset breast cancer. Higher body mass indices based on reported weights during early adulthood were also associated with reduced risk. Measures of body mass beyond the age of 20, however, were less strongly related to risk. These inconsistent patterns appeared to be explained by an effect on risk of weight gain later in life, which was related to reduced risks for young-onset breast cancer and increased risks for later disease. The effect of weight change for early-onset breast cancer was not restricted to in situ cancers and could therefore not be attributed to detection bias. The direct relationship of body mass change with older onset disease was restricted to invasive cancers, consistent with observations of poor breast cancer prognosis among obese women. Further attention on the relationship of anthropometric variables to risk of breast cancer should focus on the timing of weight gain as well as the distribution of body fat.