Obstructive sleep apnea (OSA) and obesity have been linked to systolic and diastolic dysfunction of the left ventricle. Right ventricular function is poorly understood in the 2 clinical conditions. Data from this study show that otherwise healthy obese patients with OSA had increased an left atrial volume index compared with similarly obese patients without OSA (16.3 ± 1.2 ml/m in obese patients without OSA vs 20.2 ± 1.0 ml/m in those with OSA, p = 0.02) and altered diastolic function reflected by changes in mitral annular late diastolic velocity (−5.7 ± 0.7 cm/s in obese patients without OSA vs −7.3 ± 0.7 cm/s in those with OSA, p = 0.007), mitral annular early diastolic velocity (−7.9 ± 0.6 cm/s in obese patients without OSA vs −6.4 ± 0.3 cm/s in those with OSA, p = 0.05), and early to late diastolic annular ratio >1 (82% of obese patients without OSA vs 26% of those with OSA, p = 0.001), which may be signs of early subclinical impairment of cardiac function. Importantly, healthy obese subjects had similarly increased left ventricular mass compared with obese patients with OSA but normal diastolic function and left atrial size. There was a trend toward abnormal right ventricular filling in patients with OSA, measured by altered superior vena cava diastolic velocity during expiration (−15 ± 2 cm/s in obese patients without OSA vs −10 ± 3 cm/s in those with OSA, p = 0.2) and a tendency toward diastolic dysfunction reflected by decreased lateral tricuspid annular early diastolic velocity (−7.2 ± 0.5 cm/s in obese patients without OSA vs −6.1 ± 0.5 cm/s in those with OSA, p = 0.1) beyond that seen in obesity alone. In conclusion, OSA independent of obesity may induce cardiac changes that could predispose to atrial fibrillation and heart failure.