Background: The effect of obstructive sleep apnea (OSA) on left ventricular diastolic function remains controversial. Therefore, we sought to evaluate whether severe OSA is associated with impaired diastolic function and the relationship between diastolic parameters and severity of OSA using conventional and tissue Doppler echocardiography. Methods: We studied 20 patients with severe OSA (apnea-hypopnea index, AHI ��� 30) and age-, gender-, and body mass index-matched 20 controls proved to be free of OSA (AHI < 5) on complete overnight polysomnography. All of them underwent conventional echocardiography and tissue Doppler imaging to measure LV systolic mitral annulus velocity (Sa) and early diastolic mitral annulus velocity (Ea). Results: Of all echocardiographic parameters investigated, only Ea (-6.15짹1.4 cm/s vs. -7.42짹1.4 cm/s, p = 0.005) was significantly decreased in the patients group compared with the control group, which may be a sign of early subclinical imparement of cardiac function. Other echocardiographic parameters of diastolic function such as isovolumic relaxation time (IVRT), deceleration time (DT), mitral inflow E/A ratio, and pulmonary vein S/D ratio were comparable between the two groups. AHI was correlated with the tissue Doppler imaging indexes (Ea: r = -0.453, p = 0.003; E/Ea, r = 0.323, p = 0.04) but not with conventional Doppler indexes. AHI remained a significant predictor of Ea after adjusting for age, heart rate, systolic blood pressure, body mass index, and left ventricular mass index in a multivariate stepwise regression model (p = 0.002). Conclusion: Severe OSA was associated with left ventricular diastolic dysfunction in this study. Of all echocardiographic parameters of diastolic dysfunction investigated, only Ea was identified as the best index to detect early diastolic dysfunction in patients with severe OSA.