We retrospectively evaluated 32 patients with unstable angina (UA) and no evidence of increased oxygen demand during episodes of chest pain (no significant changes in heart rate and blood pressure), who developed an acute myocardial infarction (AMI) during the same hospitalization. Based on the type of ST changes during anginal pain, two groups were defined: Group A included 19 patients who developed ST elevation during AMI; 15 of these 19 patients (79%) were in Killip class I, two were in class II, and there was one patient each in classes III and IV, respectively. Only one of the 19 patients died. Group B included 13 patients who developed ST depression during AMI; nine of these 13 patients were in Killip class IV and the remaining four patients died before they could be evaluated. Ten patients died (77%) (p less than 0.01), seven in electromechanical dissociation and three in cardiogenic shock. Postmortem examination, performed in four patients, revealed total obstruction of the left main coronary artery. It is concluded that patients with UA who, during attacks of chest pain, develop ST depression and no evidence of increased oxygen demand may have a poor prognosis when they develop an AMI. This selected group of high-risk patients appears to need immediate intensive medical care and most probably early surgical treatment.