Both depressed left ventricular ejection fraction and ventricular arrhythmias have been associated with a poor prognosis following acute myocardial infarction. To assess the relative role of each of these parameters in predicting mortality in the early period after hospitalization for myocardial infarction, 24 hour ambulatory electrocardiographic tape recordings and gated cardiac blood pool scans were obtained in 81 patients approximately two weeks after their admission to the hospital for myocardial infarction. Lown class 0 to II ventricular premature contractions during this period were classified as uncomplicated ventricular arrhythmias and Lown class III to V ventricular premature contractions were classified as complicated ventricular arrhythmias. Ejection fraction was calculated from biplane images of gated cardiac blood pool scans. In 36 patients the ejection fraction was ≥ 0.40; only three of these had complicated ventricular arrhythmias. In 45 patients the ejection fraction was < 0.40; 26 of these had complicated ventricular arrhythmias. Eight patients had documented ventricular fibrillation or instantaneous death during a mean 7.0 month (range 2 to 16 months) follow-up period outside the hospital. Although the number of patients studied was small, and there were only eight sudden deaths, life table analysis projected a one year mortality of 66 per cent in patients with complicated ventricular arrhythmias and 31 per cent in patients with an ejection fraction < 0.40. All eight patients who died suddenly were in the subgroup of 26 patients with an ejection fraction < 0.40 and complicated ventricular arrhythmias; none was in the subgroup of 19 patients with an ejection fraction < 0.40 and uncomplicated ventricular arrhythmias (p < 0.02). Although a low ejection fraction may suggest a poor prognosis following myocardial infarction, the presence of complicated ventricular arrhythmias significantly increases the risk of sudden cardiac death in the early period after hospitalization in patients with low ejection fraction.