Adults with repaired tetralogy of Fallot and significant chronic pulmonary regurgitation are at risk for progressive right ventricular (RV) dilatation and dysfunction. The assessment of RV function is important in the management in these patients. There is still a lack of an adequate geometric model to quantify RV function by echocardiography. The myocardial performance index (MPI) is a nonvolumetric method to quantify global ventricular function. In this study, the accuracy of MPI obtained by echocardiography to quantify RV function was assessed in 57 adults with repaired tetralogy of Fallot. The MPI measurement was compared with the RV ejection fraction (EF) derived by cardiac magnetic resonance imaging. There was a negative linear correlation between the MPI and the RVEF (r = 0.73, p <0.001). A MPI cutoff of ≥0.40 had a sensitivity of 81% and a specificity of 85% to diagnose a RVEF <35%. A MPI cutoff of <0.25 had a sensitivity of 70% and a specificity of 89% to identify patients with RVEFs ≥0.50. In a multivariate regression model, the MPI was not affected by the degree of pulmonary regurgitation, the presence of tricuspid regurgitation, or the QRS duration. In conclusion, the Doppler-derived MPI is a simple and reliable method for the evaluation of RV systolic function in adults with repaired tetralogy of Fallot.