In 1,395 patients admitted to hospital between 1976 and 1981 due to suspected acute myocardial infarction, the 5-year mortality rate was related to whether they developed infarction or not during the first 3 days. In all, patients with definite myocardial infarction had a 5-year mortality rate of 33.4% as compared with 13.3% in patients not fulfilling the criteria for this diagnosis (p less than 0.001). When separately analyzing patients with no previous myocardial infarction before admission and discharged from hospital, the corresponding mortality rate was 24.1% for myocardial infarction patients versus 8.1% in nonmyocardial infarction patients (p less than 0.001). Among all patients with nonconfirmed myocardial infarction, those who partly fulfilled the criteria (possible myocardial infarction) had a 5-year mortality rate of 16.7% as compared with 12.0% in those in whom myocardial infarction was completely ruled out (p = 0.18). Independent risk factors for death among patients not developing early infarction were high age and a clinical history of previous myocardial infarction and smoking. We conclude that in this study the long-term prognosis among patients admitted to hospital due to suspected acute myocardial infarction was clearly related to whether they developed an infarction or not during the first 3 days in hospital.