Higher young-adult BMI was associated with lower risk of premenopausal breast cancer both in BRCA1 (HR 0.75 per 5 kg/m2, 95% CI 0.66–0.84) and BRCA2 (HR 0.76 per 5 kg/m2, 95% CI 0.65–0.89) variant carriers. Here, BRCA1 is linked to breast cancer.