Although risk factors for acute coronary syndromes have been extensively studied, characteristics distinguishing women who will develop unstable angina rather than acute myocardial infarction (MI) are less well understood. This analysis evaluates baseline demographic, physical, and medical characteristics as predictors of angina versus MI in the Women's Health Initiative Observational Study. During a prospective 4.5-year follow-up of 92,152 postmenopausal women, 1,527 hospitalizations for angina and 797 for MI were confirmed by centrally trained physician adjudicators. In a multivariate analysis of women with incident angina or MI, high cholesterol (odds ratio [OR] 0.62, 95% confidence interval [CI] 0.47 to 0.80; p = 0.0004) and prior coronary disease (OR 0.70, 95% CI 0.55 to 0.89; p = 0.004) independently predicted angina (referent), whereas current cigarette smoking (OR 1.60, 95% CI 1.13 to 2.26; p = 0.007) and diabetes mellitus (1.44, 95% CI 1.10 to 1.87; p = 0.007) predicted MI. Older age and hypertension were independently, but less strongly, predictive of MI. Aspirin or statin use, physical activity, body mass index, and educational levels were not independently associated with one or the other type of acute coronary syndrome. Thus, specific risk factors strongly and independently predicted whether women with an acute coronary syndrome would present with angina or with MI.