The myocardial performance index (MPI) has been proposed to be a simple echocardiographic index of right ventricular (RV) function in patients after surgical repair of tetralogy of Fallot (TOF). However, its functional status remains to be clarified. The functional implications of RV MPI were determined by exploring its relationships with parameters of RV function as derived from cardiovascular magnetic resonance (CMR), and exercise capacity of postoperative TOF patients Thirty patients (11 males), aged 15.6 +/- 3.1 years, who have undergone surgical repair of TOF at 4.0 +/- 1.8 years, were studied. The RV and left ventricular (LV) MPIs determined using pulsed-wave Doppler echocardiography were related to CMR-derived RV and LV ejection fractions, and pulmonary regurgitant fraction and treadmill exercise testing parameters. Log RV MPI correlated positively with log LV MPI (r = 0.38, P = 0.037) and negatively with CMR-derived RV ejection fraction (r = -0.4, P = 0.028) and pulmonary regurgitant fraction (r = -0.4, P = 0.031). No significant correlations were found between LV MPI and any of the CMR parameters. Using receiver operated characteristics analysis, a cutoff value of 0.30 for RV MPI was found to have a sensitivity of 100% and specificity of 74% in predicting a RV ejection fraction <35%. Right ventricular, but not LV, MPI correlated inversely with exercise duration (r = -0.45, P = 0.013) and peak oxygen consumption (VO2 max) (r = -0.56, P = 0.001). Multivariate analysis identified RV MPI (beta = -0.6, P < 0.001), male sex (beta = 0.44, P = 0.01), and duration from surgery (beta = -0.30, P = 0.019) as significant determinants of VO2 max. Increased MPI is a reflection of reduced RV ejection fraction and exercise capacity in patients after TOF repair.