We assessed pulmonary function in 14 mechanically ventilated newborn very low birth weight infants with idiopathic respiratory distress syndrome by means of a face-out, volume displacement body plethysmograph and nitrogen washout analyses. Specially designed computer programs were used for calculations of lung volumes, ventilation, gas mixing efficiency, and mechanical parameters. In addition to very low compliance and moderately elevated resistance of the respiratory system, there were considerably impaired gas mixing efficiency and low functional residual capacity (FRC). No correlations between positive end-expiratory pressure and mean airway pressure versus compliance, resistance, or FRC could be found. Neither could correlations be found between FRC and compliance or FRC and the calculated right to left shunt.