Symptoms of acute severe congestive heart failure always raise the suspicion of acute myocardial infarction (AMI). Our aim was to describe the characteristics and the prognosis among patients with acute severe congestive heart failure in relation to whether or not it was caused by AMI. Consecutive patients admitted to one single hospital during 21 months due to suspected AMI, who either had initial symptoms of acute severe congestive heart failure or developed such symptoms during the subsequent days, were prospectively followed for 1 year in terms of mortality and morbidity. Of 531 patients who fulfilled previously defined criteria for severe congestive heart failure, 40% developed AMI during the first 3 days in hospital. At baseline, these patients differed from the others, having a less frequent history of known congestive heart failure. Mortality during 1 year of follow-up was 47% among patients who developed AMI versus 38% among those who did not (p < 0.05). In addition to age and a history of diabetes, development of AMI was independently associated with death.