Many patients with obstructive sleep apnea syndrome (OSAS), despite therapy with nasal continuous positive airway pressure (CPAP), have persisting daytime somnolence that may be due to a persistently elevated upper-airway resistance associated with electroencephalographic (EEG) arousals. We tested the hypothesis that elevated upper-airway resistance can be inferred from the contour of the inspiratory flow tracing obtained from a conventional CPAP circuit. This may provide a noninvasive method for determining optimal CPAP. Data were collected during a CPAP titration of an upper-airway model and in eight patients with OSAS. Estimated inspiratory resistance was calculated from esophageal pressure, CPAP mask pressure, and inspiratory flow. At high CPAP, resistance was low and inspiratory flow contour was found to be rounded. At low CPAP, resistance was high and flow contour developed a plateau suggesting flow limitation. We also noted that the CPAP pressure at which high resistance developed, and at which flow limitation appeared, showed hysteresis. We conclude that when respiration is stable, the contour of inspiratory flow tracing from a CPAP system can be used to infer the presence of elevated upper-airway resistance and flow limitation. Optimizing flow contour may be an alternative to eliminating apneas in evaluation of the optimal therapeutic level of CPAP in OSAS.