Respiratory system compliance (Crs) was measured in 34 spontaneously breathing infants during the first year of life. An occlusion technique was used whereby several expiratory occlusions were performed at different lung volumes within the tidal range. The airway opening pressure generated during a plateau after occlusion was related to the volume included above the end-tidal level by a regression equation. The slope of this equation represented the compliance of the infant's respiratory system; the intercept was significantly different for preterm (-0.5 ml) and post-term (-5.5 ml) infants and may represent the difference between end-expiratory lung volume during tidal breathing and the relaxed functional residual capacity. The values for respiratory system compliance were similar to those previously reported for infants during muscle relaxation. As a function of body length, Crs = 1.58 X length3.13 X 10(-4) ml . kPa-1. The technique described is simple to apply and is independent of oesophageal pressure measurements.