Objective: The myocardial performance index (MPI) enables noninvasive estimation of combined systolic and diastolic function. The only di- astolic function parameter used in the index is the isovolumic relaxation time (IRT). We assessed the impact of shortened IRT in restrictive filling pattern on MPI. Materials and Methods: The MPI was defined as the sum of the isovolumic contraction time (ICT) and the IRT divided by the ejection time (ET), and was calculated in 13 controls (group 1), and 39 patients with ischemic heart disease with left ventricular systolic dysfunction (ejection fracti- on<50%). The patients were classified into two groups according to mitral early filling deceleration time (DT): group 2 with DT>140 ms (n = 20), and group 3 with DT ≤140 ms (n = 19). Results: The ICT was longer and the ET was shorter in group 2 and group 3 than in group 1. The ICT and ET were not different in group 2 and group 3. The IRT was longer in group 2 (130±23 ms) compared to group 1 (82±10 ms, p<0.001) and group 3 (85±19 ms, p<0.001), but did not differ between group 1 and group 3. The MPI was higher in group 2 and group 3 compared to group 1 (0.79±0.25 and 0.65±0.19 vs 0.42±0.08, p<0.001 and p<0.001; respectively). However, it was lower in group 3 than in group 2 (p=0.03) due to significantly shortened IRT. Conclusion: Shortened IRT in patients with restrictive filling pattern results in reduction of the MPI. Therefore, the MPI may not reflect true level of ventricular dysfunction in these patients.