We evaluated the associations of body mass index (BMI) at age 18 years and recent BMI in relation to risk of breast cancer overall and ER/PR/HER2-defined subtypes, in 6320 women (3934 case-patient participants, 2386 control participants) aged 35–64 years, who participated in one of three population-based case-control studies. Here, ESR1 is linked to breast cancer.