To assess interactions between use of estrogen plus progestin hormone therapy (EPHT) and body mass index (BMI) in relation to risks of different types of breast cancer, based on histology and hormone receptor status.

We conducted a population-based case-control study that compared 975 postmenopausal breast cancer cases to 1,007 controls. Interactions between menopausal hormone therapy (HT) and BMI in relation to risk of different breast cancer types were evaluated using logistic regression.

Obese (BMI ≥30.0 kg/m²) never users of HT had 1.7–fold to 2.3–fold elevated risks of ductal and ER+/PR+ tumors, respectively, compared to thinner women. BMI was not related to breast cancer risk among current HT users. Current EPHT users for ≥5 years had 2.1 to 9.6-fold elevated risks of lobular and ER+/PR+ tumors compared to never users of HT regardless of BMI. Current EPHT users for ≥5 years with a BMI ≤24.9 kg/m² also had a 2.6-fold elevated risk of ductal carcinoma. However, none of the interactions between BMI and HT use evaluated reached statistical significance.

While interactions between HT and BMI are well established, they appear to only be present among certain breast cancer types. Since obesity is related to breast cancer risk only among never users of HT, as HT use declines the relative impact of obesity on breast cancer incidence may grow.