We investigated prior to gastric bypass surgery the prevalence of left ventricular diastolic dysfunction (LVDD) by Doppler and tissue Doppler echocardiography in 14 obese women and in 6 obese men, mean age 45 years, with a mean body mass index of 49 ± 5 kg/m<sup>2</sup> who had nocturnal polysomnography for obstructive sleep apnea (OSA). The Doppler and tissue Doppler echocardiographic data were analyzed blindly without knowledge of the clinical characteristics or whether OSA was present or absent. Of 20 patients, 8 (40%) had no OSA, 4 (20%) had mild OSA, and 8 (40%) had moderate or severe OSA. Moderate or severe LVDD was present in 4 of 8 patients (50%) with moderate or severe OSA and in none of 12 patients (0%) with no or mild OSA (p < 0.01). Obese patients with moderate or severe OSA have a higher prevalence of moderate or severe LVDD than obese patients with no or mild OSA.