Seventeen men (age range, 21 to 58 years) with severe obstructive sleep apnea syndrome (OSAS) documented by nocturnal polysomnography underwent hemodynamic investigations during sleep. We studied the evolution of the cardiac index during apneic events in both NREM and REM sleep using the thermodilution technique. Regardless of the patient's age, the sleep state, or the mechanism inducing the OSAS, the cardiac index decreased significantly from baseline during an apneic event and increased significantly from baseline at the resumption of ventilation.