In 309 female ER+ breast cancer patients being treated with aromatase inhibitors, smoking before surgical treatment for their disease was associated with increased numbers of breast cancer events (e.g., recurrence, new breast cancer diagnosis, metastasis; HR = 2.97; 95% CI = 1.44–6.13), distant metastases (HR = 4.19; 95% CI = 1.81–9.72), and mortality (HR = 3.52; 95% CI = 1.59–7.61). The gene discussed is CYP19A1; the disease is breast cancer.