Background -Assessment of systemic right ventricular (RV)-function is a key point in the follow-up of patients with transposition of the great arteries (TG A). Current echocardiographic assessment of RV function is at best an estimate, and cardiac magnetic resonance (CMR) is considered the gold standard. However, t his technique is expensive, has limited availability, and requires significant e xpertise to acquire and interpret the images. The myocardial performance index ( MPI)-has recently been studied for assessment of pulmonary RV function and show s promise as a simple yet powerful tool for assessing patients with RV dysfuncti on of various origins. We set out to compare MPI and CMR assessment of systemic RV function in patients with TGA. Methods and Results -Data from patients with TGA (11 with congenitally corrected TGA, 18 with surgically corrected TGA) who h ad CMR within 6 months of their echocardiogram were reviewed. The average system ic RV ejection fraction (RVEF)byCMR was 39.4±11.4%, and the systemic RVMPI for this group was 0.56±0.21. There was a strong negative correlation between the systemic RVMPI and systemic RVEF by CMR (r=-0.82, P< 0.01). The systemic RVEF c an be estimated from this formula: RVEF=65%-(45.2XMPI). Conclusions -MPI can be used in patients with systemic RVs to assess global function andtoestimatean EF with good accuracy. Background-Assessment of systemic right ventricular (RV) -function is a key point in the follow-up of patients with transposition of the great arteries (TG A). Current echocardiographic assessment of RV function is at best an estimate, and cardiac magnetic resonance However, t his technique is expensive, has limited availability, and requires significant e xpertise to acquire and interpret the images. The myocardial performance index (MPI) -has recently been studied for assessment of pulmonary RV function and show s promise as a simple yet powerful tool for assessing patients with RV dysfuncti on of various origins. We set out to compare MPI and CMR assessment of systemic RV function in patients with TGA. Methods and Results-Data from patients with TGA ( 11 with congenitally corrected TGA, 18 with surgically corrected TGA) who h ad CMR within 6 months of their echocardiogram were reviewed. The average system ic RV ejection fraction (RVEF) by CMR was 39.4 ± 11.4%, and the systemic RVMPI for this group was 0.56 ± 0.21. There was a strong negative correlation between the systemic RVMPI and systemic RVEF by CMR (r = -0.82, P <0.01). The systemic RVEF c an be estimated from this formula: RVEF = 65% - (45.2XMPI). Conclusions -MPI can be used in patients with systemic RVs to assess global function and toestimatean EF with good accuracy.