Tamoxifen prevents recurrence after breast cancer and breast cancer among high-risk women, and may prevent myocardial infarction (MI). To assess the impact of tamoxifen on MI risk, we conducted a case-control study of first MI after breast cancer nested among women diagnosed with breast cancer, while enrolled in a health maintenance organisation from 1980 to 2000. We obtained information on breast cancer treatment and MI risk factors through medical record reviews and interviews. Data were analysed using conditional logistic regression. Of 11,045 women with breast cancer, 134 met MI criteria and were matched to two MI-free control subjects on year of birth and breast cancer diagnosis. After adjusting for smoking, hypertension and diabetes, tamoxifen was unassociated with MI (odds ratio (OR)=1.2, 95% confidence interval (CI)=0.7-1.9). Duration, cumulative dose and recency of use were not associated with MI. Radiation therapy was associated with MI (OR=2.0, 95% CI=1.1-3.5), an association that varied slightly but not statistically significantly by tamoxifen use (radiation with tamoxifen, OR=2.0, 95% CI=0.9-4.4; radiation without tamoxifen, OR=2.9, 95% CI=1.2-7.5). Tamoxifen treatment for breast cancer does not appear to increase or decrease MI risk, although radiation therapy appears to increase MI risk.