Respiratory system compliance (Crs) can be used to assess lung stiffness in sick, intubated infants, avoiding the use of an esophageal balloon. Crs was assessed in a group of 15 sick, intubated infants using the occlusion and inflation techniques. The occlusion technique gave satisfactory results in 13 infants. Apneic pauses following occlusion were obtained in infants up to 10 months of age. Satisfactory measurements of Crs were obtained in all 15 infants using the inflation technique, but difficulty was experienced in obtaining data over the tidal volume range in three of them. A close agreement was found between the two methods of measuring Crs in the ten infants in whom a direct comparison was possible.