The Tei index is a Doppler-derived myocardial performance index. It is a measure of the combined systolic and diastolic myocardial performance of both the left and right ventricles. The incidence of heart failure (HF) is increasing globally, and its severity can be clinically assessed using the New York Heart Association (NYHA) functional classification and more objectively using echocardiographic assessment of systolic and diastolic functions. Thus, a measure of the combined systolic and diastolic myocardial performance could be a useful predictor of the severity of the clinical status of patients with HF. Seventy-five newly presenting patients with HF of NYHA class II to IV and 60 normal controls were consecutively recruited. Using conventional two-dimensional and Doppler echocardiography techniques, the left ventricular parameters assessed were the isovolumic relaxation time (IVRT), isovolumic contraction time (IVCT), ejection time (ET), ejection fraction (EF), and end-diastolic volume (EDV). The Tei index was determined using the formula IVCT + IVRT/ET. The mean Tei index of patients was significantly higher than that of controls (0.884 ± 0.321 vs. 0.842 ± 0.14; p < 0.001). The Tei index ranged from 0.33 to 1.94 in patients and from 0.56 to 1.24 in controls. The mean EF was lower in patients than in controls (50.47% ± 19.01% vs. 68.35% ± 7.75%; p = 0.001). The mean EDV was higher in patients than in controls (171.39 ± 100.96 vs. 94.15 ± 28.54; p < 0.001). Comparison of the mean Tei indices of patients with HF of NYHA classes II, III, and IV showed statistically significant differences among all three groups (p < 0.001). The Tei index seems to be a clinically relevant indicator of cardiac function. It is reflective of the severity of HF as clinically assessed using the NYHA functional classification in patients with HF.