Unstable angina and non-Q-wave myocardial infarction (MI) are common and costly clinical events, but there is considerable uncertainty about optimal clinical management of these syndromes. We developed a prediction rule to help clinicians determine which patients with unstable angina or non-Q-wave MI are likely to "fail" medical therapy and ultimately require cardiac catheterization within 6 weeks of presentation. Subjects were 733 patients presenting with unstable angina or non-Q-wave MI enrolled in the TIMI IIIB trial and randomized to initial medical management. We developed a prediction rule based on logistic regression analysis of baseline data from history, physical examination, electrocardiogram, and blood studies. The outcome of interest was "failure" of medical therapy, defined as need for coronary catheterization within 42 days. Significant predictors of "failing" medical therapy included ST segment depression >or= .1 mV (odds ratio, OR, = 2.7, 95% confidence interval, CI, 1.8-4.1), accelerated angina in the prior 2 months (OR = 1.8, 95% CI 1.2-2.6), nitrate use in the prior week (OR = 1.6, 95% CI 1.1-2.2), exertional angina in the prior 2 months (OR = 1.6, 95% CI 1.1-2.2), and cardiac troponin I (cTnI) >or= 0.4 ng/mL (OR = 1.4, 95% CI 1.1-1.9). We used these variables to build a risk score by assigning point values based on these ORs. The risk score had a moderate ability to predict which patients would subsequently fail medical therapy and undergo cardiac catheterization (c = 0.682). Out of a total risk score of 13, failure of medical therapy occurred in 86% of patients who had a risk score >or= 8 (n = 111), 78% of patients who had a risk score >or= 6 (n = 240), and 72% of patients who had a risk score >or= 4 (n = 438). At scores of < 2 (n = 88), 40% of patients failed medical therapy. Although the management of unstable angina is in constant evolution, clinicians will always be faced with determining which patients should be managed most invasively. The simple prediction rule we present can be applied to patients with unstable angina or non-Q-wave MI at the time of presentation to predict which patients have a high probability of failing medical therapy. Such a rule may be useful for identifying patients who should be considered for early cardiac catheterization.