Obstructive sleep apnea (OSA) is associated with cardiovascular mortality and morbidity. It may predispose patients to left ventricular hypertrophy and heart failure. The aim of this study was to determine the left ventricular mass (LVM) and myocardial performance index (MPI) reflecting left ventricular global function in uncomplicated OSA patients. Sixty-four subjects without hypertension, diabetes mellitus, and any cardiac or pulmonary disease referred for evaluation of OSA underwent overnight polysomnography and complete echocardiographic assessment. According to the apnea hypopnea index (AHI), subjects were divided into three groups: group 1, control subjects with nonapneic snorers (AHI < 5, n = 18); group 2, patients with mild to moderate OSA (AHI: 5-30, n = 25); and group 3, severe OSA (AHI > 30, n = 21). Basic echocardiographic measurements, LVM, and LVM index were measured. Left ventricular MPI was calculated as (isovolumic contraction time+isovolumic relaxation time)/aortic ejection time by Doppler echocardiography. There were no significant differences in age, sex, body mass index, heart rate, and systolic and diastolic blood pressure among the three groups. Left atrium, interventricular septum, left ventricular posterior wall, left ventricular end-diastolic and end-systolic diameters, LVM mass, and LVM index were not significantly different among the three groups. Left ventricular MPI was significantly higher in severe OSA patients (0.64 +/- 0.18) than in controls (0.49 +/- 0.18; P < 0.05). There was no significant difference between controls (0.49 +/- 0.18) and mild to moderate OSA (0.61 +/- 0.16; P = 0.08) and between mild to moderate OSA (0.61 +/- 0.16) and severe OSA (0.64 +/- 0.18; P = 0.84). The present study demonstrates that patients with severe OSA have global left ventricular dysfunction.