Dynamic pulmonary compliance was measured in adult patients, first awake and then during halothane anesthesia under various conditions. Patients were free from respiratory obstruction and excessive inflation of the lungs during all measurements. Compliance decreased approximately one-third during deep anesthesia. During light anesthesia and emergence when tidal exchange approximated control values, there was less decrease in compliance. The occurrence of pulmonary atelectasis during deep anesthesia in spontaneously breathing patients is suggested. This may be the primary reason for decreased compliance during anesthesia. Compliance decreased in the conscious state in changing from the sitting to supine position. Men had higher measurements both awake and anesthetized than women, and greater decreases during anesthesia. A thiopental induction was associated with more of a decreased compliance than an inhalation induction. Thin patients encountered little change during anesthesia compared with normal and obese patients. During positive-pressure breathing compliance was lower than measurements during spontaneous respiration.