This prospective hospital-based, case-control study compares the outcome of unstable angina in non-insulin dependent diabetic patients and non-diabetic control subjects. One hundred and sixty-two diabetic patients and 162 non-diabetic control patients with unstable angina were entered into the study. The 3-month mortality was 8.6% (95% confidence interval, CI = 4.4-12.9%) in diabetic patients and 2.5% (CI = 0.1-4.9%) in control patients (p = 0.014). The 1-year mortality was 16.7% (CI = 10.9%-22.4%) in diabetic patients and 8.6% (CI = 4.4%-12.9%) in non-diabetic patients (p = 0.029). Diabetic patients received beta-blockade and underwent coronary angiography and angioplasty less frequently than controls; the frequency of unstable angina, of acute myocardial infarction, and coronary artery bypass grafting was similar in both groups at 1 year of follow-up. It is concluded that diabetic patients with unstable angina have a higher mortality than non-diabetic patients and that this difference is largely accounted for by early (first 3 months) mortality.