Key PointsQuestionWhat is the association between body mass index and risk for breast cancer diagnosed before menopause?FindingIn this large pooled analysis of data on 758 592 premenopausal women, an inverse association of breast cancer risk with body mass index at 18 through 54 years of age was found, most strongly for body mass index at ages 18 through 24 years. The inverse association was strongest for hormone receptor–positive breast cancer, was evident across the entire distribution of body mass index, and did not materially vary by attained age or other characteristics of women.MeaningIncreased adiposity, in particular during early adulthood, may be associated with reductions in the risk of premenopausal breast cancer.