Background The mammary gland is largely undifferentiated before birth and may be particularly susceptible to intrauterine influences that could increase the risk of cancer through acceleration of cell proliferation or other pregnancy-related processes. Studies of migrant populations, animal data, and limited epidemiological evidence suggest that breast cancer may originate in utero. In a nested case-control study we assessed whether birthweight and other perinatal factors are associated with risk of breast cancer. Methods This case-control study was nested within the cohorts of the two Nurses' Health Studies. We used self-administered questionnaires to obtain information from the mothers of 582 nurses with invasive breast cancer and the mothers of 1569 nurses who did not have breast cancer (controls). Information on risk factors for breast cancer during adulthood were obtained from the nurses; multiple logistic regression analysis adjusted for these risk factors. Findings Birthweight was a significant predictor of breast-cancer risk. With women who weighed 4000 g or more at birth as the reference category, the adjusted odds ratios for breast cancer were 0.86 (95% CI 0.59-1.25) for birthweights of 3500-3999 g, 0.68 (0.48-0.97) for birthweights of 3000-3499 g, 0.66 (0.45-0.98) for birthweights of 2500-2999 g, and 0.55 (0.33-0.93) for birthweights below 2500 g (p for trend 0.004). Prematurity was not significantly associated with risk of breast cancer. Interpretation Birthweight is significantly associated with breast-cancer risk, which suggests that intrauterine factors or processes affect the risk of breast cancer in the offspring. High concentrations of pregnancy oestrogens may have an important role in breast carcinogenesis, but other pregnancy hormones or intrauterine factors may also be involved.