Objective: The index of myocardial performance (IMP) is a global cardiac function index with prognostic utility in patients with myocar- dial infarction and dilated cardiomyopathy but is preload dependent. We hypothesized that a volume overload lesion prolonging LV ejec- tion time (LVET) may reduce IMP despite LV dysfunction (LVD). Materials and Methods: The study groups consisted of 35 normals, 26 with LV dysfunction, and 60 with aortic regurgitation (AR): 40 with ejection fracti- on (EF) >50% (AR+Normal EF) and 20 with ejection fraction ≥50% (AR+Reduced EF). We evaluated consecutive patients in each group with technically adequate 2D and Doppler echocardiography. Results: When compared to normal subjects (0.357±0.122), IMP was increased with LVD (0.604±0.278 p<0.001) but was similar in AR+Nor- mal EF patients due to isovolumic relaxation time (IRT) and LVET prolongation. The IMP was lower in AR+Reduced EF group (0.346±0.172, p<0.001) as compared to the LVD group due to a prolonged LVET and a reduced IRT and isovolumic contraction time (ICT). Conclusions: The IMP in AR+Normal EF patients was similar to normals due to IRT and LVET prolongation. The IMP was reduced in AR+Reduced EF patients compared to LVD patients due to IRT and ICT shortening and LVET prolongation. The index of myocardial per- formance in AR patients should be applied with caution.