Tamoxifen, a synthetic, non-steroidal antiestrogen in breast tissue, has been successfully used to treat breast cancer and is now being evaluated as a breast cancer chemopreventive agent. Tamoxifen produces some estrogen-like effects. It has a favorable impact on lipid profiles, but whether it reduces risk of coronary heart disease and stroke or increases risk of venous thromboembolic events (VTE: deep venous thrombophlebitis and pulmonary embolism) is uncertain. In the U.S., the Breast Cancer Prevention Trial was designed to address heart disease and stroke prevention, but recruitment has been suboptimal due to lower than expected subject accrual especially among older women. Thus, the trial lacks statistical power to pursue these issues. As tamoxifen is already widely used in the treatment of breast cancer and may become widely used in disease-free women, the investigators note that it is important to assess its potential risks and benefits. The overall objectives of this HMO-based case-control study are as follow: 1) to determine whether tamoxifen treatment for breast cancer reduces the risk of incident myocardial infarction (MI) and stroke and increases the risk of incident VTE; 2) to determine whether any tamoxifen effects are restricted to current users and to quantify any dose, duration or latency effects; and 3) to determine whether any observed effects of tamoxifen on risk of these events are explained by or modified by relevant disease risk factors. Cases will be breast cancer patients diagnosed at a Los Angeles County Kaiser Permanente Southern California (KPSC) health care centers who subsequently are diagnosed with incident MI, stroke or VTE. Cases will be identified by linking data from the population-based cancer registry for Los Angeles County to the KPSC discharge database. Two controls will be individually matched to each case. Controls will be KPSC members diagnosed with breast cancer who have not had the case-defining event, who matched the cases on year of diagnosis and year of birth within 3 years. Neither cases nor controls can have had other cancer diagnoses except a second primary breast cancer. Controls must be alive at the time their matched case experienced her case-defining event. Breast cancer treatment and relevant risk factor data will be collected and analyzed for 160 breast cancer patients diagnosed with MI and 320 controls, for 260 breast cancer patients diagnosed with stroke and 520 controls, and 210 breast cancer patients diagnosed with VTE and 420 controls.