The aim of this study was to investigate the impact of obstructive sleep apnea (OSA) on left ventricular (LV) functional changes by using tissue Doppler imaging–derived indexes in patients with OSA. We studied 62 patients classified into 3 groups, namely 18 with mild to moderate OSA, 24 with severe OSA, and 20 control subjects without OSA according to the apnea–hypopnea index (AHI) on complete overnight polysomnogram. All underwent conventional and tissue Doppler echocardiographies. Only early diastolic velocity (Ea; −6.2 ± 0.3 vs −7.1 ± 0.3 vs −7.3 ± 0.3 cm/s, respectively, for the 3 groups, p = 0.023) was significantly decreased in the severe OSA group. Other echocardiographic parameters of diastolic function such as isovolumic relaxation time, deceleration time, mitral inflow early/late wave velocity ratio, and pulmonary vein systolic/diastolic pulmonary vein velocity ratio were comparable among the 3 groups. AHI was correlated only with tissue Doppler imaging–derived indexes of LV diastolic function (Ea r = −0.382, p = 0.002; Ea/late diastolic velocity r = −0.329, p = 0.009), but not with conventional Doppler indexes. AHI remained a significant predictor of Ea after adjusting for age, heart rate, fasting glucose level, blood pressure, body mass index, and LV mass index in a multiple stepwise linear regression model (p = 0.007). In conclusion, only patients with severe OSA showed a greater impairment of LV diastolic function. Of all echocardiographic parameters of diastolic dysfunction investigated, only Ea was identified as the best index to demonstrate an association between LV diastolic dysfunction and severity of OSA independently of body mass index, diabetes mellitus, and hypertension.