A cross-sectional survey involving 51 children, ranging in age from 3 wk to 15 yr, was performed to examine the changes in respiratory mechanics with age in mechanically ventilated children, using both a single-compartment model of the respiratory system and a more sophisticated two-compartment model. Children were studied while under anesthesia for urological surgery and were considered to have normal lungs. They were paralyzed and mechanically ventilated throughout measurements. Respiratory mechanics were measured during ventilation by applying a single-compartment model and by using multilinear regression to calculate dynamic compliance and respiratory system resistance (Rrs). We then used the interrupter technique, which allowed us to partition Rrs into airway resistance and a tissue viscoelastic component known as Pdif. A static volume-pressure curve was constructed from multiple occlusions made at different lung volumes throughout expiration, and static compliance was determined. Rrs and airway resistance decreased as height increased. There was a progressive increase in respiratory system compliance with height. Pdif fell in the first 2 yr of life and then subsequently increased after the age of approximately 5 yr.