Left ventricular stroke volume (LVSV) falls during obstructed inspiration in animals and normal human subjects through mechanisms that may be closely related to pleural pressure. In this study we postulated that a similar reduction in LVSV should occur in patients with obstructive sleep apnea (OSA). Daytime polysomnograms were performed in 10 patients with OSA. A noninvasive electrical impedance method was used to determine LVSV. Pleural pressure was measured by esophageal balloon. In comparison with awake values, during OSA we found reductions in LVSV, cardiac output, and heart rate of 18, 27, and 11%, respectively (P less than 0.01). We observed that systolic pleural pressure did not have a significant effect on LVSV (P greater than 0.05). However, at pleural pressures lower than 10 cmH2O below resting expiratory level, there was a linear relationship between falls in LVSV and falls in middiastolic pleural pressure (P less than 0.0001). We concluded that reduced LVSV shown in patients with OSA was significantly related to diastolic pleural pressure level. Our findings suggested reduced preload as the most likely mechanism for decreased cardiac output in OSA.