Management of Q-wave acute myocardial infarction (AMI) has been shown to differ between the United States and Canada, with more catheterization and revascularization procedures performed in the United States, but with little or no apparent difference in clinical outcomes. No previous studies have evaluated management differences for the acute coronary syndromes of unstable angina pectoris and non-Q-wave AMI. We therefore compared treatments and outcomes between 14 United States and 4 Canadian tertiary care centers participating in an observational registry of all consecutive admissions for unstable angina or non-Q-wave AMI between 1990 and 1993. A random, stratified sample was selected for detailed assessment and follow-up. There were 1,733 patients enrolled in United States centers and 642 in Canadian ones. In United States centers patients were less likely to receive intravenous nitroglycerin, heparin, beta blockers, calcium antagonists, or > or = 2 anti-ischemic agents. Coronary arteriography during index hospitalization was equally frequent in both countries (63.4% vs 66.9%, p = 0.781), but at 6 weeks and 1 year coronary arteriography was slightly less frequent in the United States patients. Revascularization by coronary angioplasty or bypass surgery was equivalent at 6 weeks and 1 year; however, there were trends toward less angioplasty and more bypass surgery in the United States than in Canada. Patients at United States centers stayed in the hospital fewer days than patients at Canadian centers (mean 8.2 vs 12.1 days, p <0.001). Death or AMI by 6 weeks was not different (4.8% vs 4.4%, p = 0.633), nor was it different at 1 year (10.0% vs 10.2%, p = 0.836). The combined outcome of death, AMI, or recurrent ischemia was more common in United States than in Canadian patients at 6 weeks (18.4% vs 13.9%, p = 0.004). Our findings indicate that United States physicians and hospitals did not consistently utilize more resources and were not more aggressive than their Canadian counterparts when treating acute coronary syndromes during this period.