It is generally accepted that tamoxifen, but not aromatase inhibitors, is associated with an increased risk of thrombosis in women with breast cancer. Here, we report an unusual case of an 87-year-old female on anastrozole therapy with aortic thrombus extending into the left subclavian artery with associated diffuse venous thromboembolism (VTE). The gene discussed is CYP19A1; the disease is deep vein thrombosis.