A 1 category increase in passive smoking intensity was associated with an OR of 1.56 (95% CI: 1.28–1.89; Ptrend < 0.001) for ER+/PR- breast cancer, which was different from the passive smoking/ER-/PR- association (Pheterogeneity = 0.05), with an OR of 1.34 (95% CI: 1.15–1.57; Ptrend < 0.001) for ER+/PR+ breast cancer, not significantly different from the passive smoking/ER-/PR- association (pheterogeneity = 0.35), and with an OR of 1.20 (95% CI: 1.00–1.44; Ptrend = 0.05) for ER-/PR- breast cancer. This evidence concerns the gene ESR1 and breast cancer.