A series of 89 surgical patients (111 operations) with preoperative myocardial infarction (MI) was analysed for factors predisposing to the development of a postoperative reinfarction. Six of them suffered postoperative MI, and three of these patients died. In the statistical analysis the following risk factors emerged: age over 60 years, anaemia, hypertension, and the fact that the previous MI had been posterior. Abdominal operations were more dangerous concerning reinfarction than other operations. In the other series of 11 deceased patients with postoperative reinfarction collected from the autopsy material, about the same risk factors were found. The most important factor seemed to be hypotension, which complicated the surgery. All 11 patients had arrhythmias in their preoperative electrocardiogram. Previously treated heart failure was present in five of these patients. Postoperative symptoms analysed in the first series suggest that if a patient with preoperative MI has arrhythmias, hypotension, dyspnoea, diffuse unlocalized pain or chest pain after surgery, he is very likely to have a reinfarction.