Of 413 patients with in-hospital ventricular fibrillation complicating acute myocardial infarction 281 had early (less than 48 hours post infarction) ventricular fibrillation and 132 had late ventricular fibrillation. In-hospital and longterm prognoses were not influenced by time of ventricular fibrillation, but advanced age and signs of reduced left ventricular function were found to be independent significant risk factors for short and longterm prognoses.