For patients ≥ 50 years with ER+ tumors, a waist circumference ≥ 80 cm was associated with a shorter breast cancer-free interval if tamoxifen-treated (Log-Rank p = 0.048; adjHR 2.05; 95% CI 1.01–4.15) or AIs (Log-Rank p = 0.027; adjHR 2.82; 95% CI 0.99–8.04). This evidence concerns the gene ESR1 and breast carcinoma.