Purpose - To determine the prognostic significance of several parameters of left ventricular mechanics and of complex ventricular arrhythmias. Patients and Methods - 184 chronic chagasic and 85 primary dilated cardiomyopathy patients were submitted to non-invasive and invasive studies, including Holter monitoring and “M” mode echocardiogram and followed for a maximum of 8 years. Results - Finding of a depressed left ventricle (LV) systolic function (% AC) was the most important independant prognostic factor in both groups of patients. The presence of complex ventricular arrhythmias had an additional, but very limited, independent prognostic importance, observed only in chagasic patients with depressed LV systolic function. When patients with similar degress of left ventricular systolic dysfunction, inadequate hypertrophy or increased afterload were compared, chagasic patients had significantly worse prognosis than those with primary dilated cardiomyopathy. This is partly atribuited to higher prevalence of complex ventricular arrhythmias. Conclusion - The higher prevalence of complex ventricular arrhythmias in patients with chronic Chagas’ disease may explain the prognostic differences between the groups.