The interrupter technique involves measuring pressure changes at the airway opening following abrupt interruption of airflow, and relies on rapid equilibration of alveolar and airway opening pressures. Following airway occlusion one generally sees a rapid change in airway opening pressure, Pinit, which reflects the resistive pressure drop across the system, followed by a secondary, slower pressure change, Pdif, which reflects the tissue visco-elastic properties together with any redistribution of gases occurring between lung units at different pressure at the time of interruption. Physical factors in the measuring equipment, i.e., the time taken for valve closure and the presence of a proximal compliant compartment, can cause errors in the calculation of resistance and elastance using the interrupter technique. A computer model was used to determine the effects of these factors on the calculation of resistance and elastance in normal infants, premature infants, and infants with bronchopulmonary dysplasia. The elastance from 25 ml of dead space gas did not introduce significant errors into the measurements in any group of infants, but the presence of an unsupported upper airway caused errors of up to 245%. Increasing valve closure time introduced progressively larger errors into the calculation of resistance and elastance. To keep these errors acceptably small, an occlusion valve should close in 20 ms or less.