Thoracoabdominal motion (TAM) profiles were determined in ten infants requiring nasal continuous positive airway pressure (CPAP) and supplemental oxygen, in order to assess the influence of CPAP on chest wall function in infants with respiratory insufficiency. (TAM) was quantitated by respiratory inductive plethysmography, measuring relative motion of the rib cage and abdomen. Baseline pulmonary function (without CPAP support) was assessed from simultaneous measurements of transpulmonary pressure, air flow, and tidal volume. Measurements of (TAM) were acquired at baseline and at nasal CPAP levels of 0, 3, 5, and 8 cm H2O, in randomized order. Without CPAP, relative paradoxical motion occurred, i.e., the rib cage collapsed inward instead of expanding outward early in inspiration. With CPAP, TAM resembled the pattern in preterm infants, without lung disease. We found that nasal CPAP lowers the phase angle in infants with respiratory insufficiency (P less than 0.003), indicating improved synchrony of TAM. In addition, the improvement with nasal CPAP was related to the severity of pulmonary compromise at baseline. We speculate that changes in TAM associated with nasal CPAP arise from an interaction between pulmonary mechanics and an enhanced stability of the chest wall. In this context, the greater synchrony of TAM is suggestive of an improved breathing strategy. This may be a noninvasively obtainable marker of an effective nasal CPAP level in infants with altered pulmonary and chest wall mechanics.