Objectives. The purpose of the study was to analyze the factors that favor the occurrence of sustained monomorphic ventricular tachycardia in the early phase (<48 h) of acute myocardial infarction and to establish its prognostic implications. Background. Sustained monomorphic ventricular tachycardia early in the course of an acute myocardial infarction is an uncommon arrhythmia, and its significance has not been specifically studied. Methods. The clinical characteristics and prognosis of sustained monomorphic ventricular tachycardia were studied in 21 (1.9%) of 1,120 consecutive patients admitted to the coronary care unit with a diagnosis of myocardial infarction. Results. Patients with sustained monomorphic ventricular tachycardia had a larger infarct on the basis of peak creatine kinase, MB fraction (CK-MB) isoenzyme activity (435 ± 253 IU/liter vs. 168 ± 145 IU/liter, p < 0.001) and higher mortality rate (43% vs. 11%, p < 0.001). By logistic regression analysis, independent predictors of sustained monomorphic ventricular tachycardia were CK-MB (odds ratio [OR] 11.8), Killip class (OR 4.0) and bifascicular bundle branch block (OR 3.1). Moreover, sustained monomorphic ventricular tachycardia was itself an independent predictor of mortality (OR 5.0). Compared with patients with ventricular fibrillation, those with sustained monomorphic ventricular tachycardia had a worse Killip class (Killip class >I: 63% vs. 30%, p < 0.05), higher CK-MB activity (430 ± 260 IU/liter vs. 242 ± 176 IU/liter, p < 0.01) and higher arrhythmia recurrence rate (31% vs. 4%, p < 0.01). During the follow-up period, 5 (42%) of 12 survivors in the sustained monomorphic ventricular tachycardia group died of cardiac-related causes. Recurrence of ventricular tachycardia was seen in two patients (17%). Conclusions. Sustained monomorphic ventricular tachycardia during the first 48 h of myocardial infarction is a sign of extensive myocardial damage and an independent predictor of in-hospital mortality.