The index of myocardial performance (MPI) has been used as an easily obtainable parameter that reflects both systolic and diastolic functions of the myocardium and correlates closely with invasive measurements. This study investigated the importance of MPI on assessment of left ventricular function in patients with critical coronary artery disease. Methods: We studied 82 patients who had coronary angiography and echocardiography. Patients in Group A were without critical coronary stenosis (n = 37, 17 females, 20 males, mean age 54 +/- 11 years) and patients in Group B had critical coronary stenosis (> 70%) without previous myocardial infarction (n = 45, 18 female, 27 male, mean age 57 +/- 10 years). Using echocardiographic parameters, left ventricular isovolumetric relaxation time (IRT), isovolumetric contraction time (ICT), ejection time (ET), ratio of velocity time integrals (vti) of early and late diastolic mitral flow (E/Avti), E deceleration time (EDT), MPI [(IRT + ICT) / ET], ejection fraction (EF), and fractional shortening (FS) were calculated. During cardiac catheterization, Dp/Dt [(diastolic blood pressure - left ventricular end diastolic pressure) / ICT] was calculated. Results: There were significant differences in IRT, EDT, E/Avti, and the MPI between Groups A and B (95.9 +/- 14.7 and 113.4 +/- 14.3 msec, P < 0.001; 164.5 +/- 44.8 and 186.2 +/- 33.6 msec, P < 0.05; 1.51 +/- 0.45 and 1.24 +/- 0.80 msec, P < 0.05; and 0.45 +/- 0.08 and 0.53 +/- 0.07 msec, P < 0.001, respectively), but there were no significant differences in ICT, ET, EF, FS, and Dp/Dt between the two groups. Both groups showed a close correlation between MPI and Dp/Dt (r = - 0.78 for Group A and r = - 0.82 for Group B). There were no significant differences in heart rate and systolic and diastolic blood pressure between the two groups. Conclusion: These data suggest that MPI may be a useful parameter and an early indicator of left ventricular dysfunction in patients with critical coronary artery disease and normal systolic function.