The effects of the inhalational anaesthetic, isoflurane, on two major parameters of respiratory mechanics--resistance and compliance--were studied in a total of 30 patients. With increasing inspiratory concentrations of isoflurane, resistance was measured in 5 spontaneously breathing patients using the oscillation method. In 16 mechanically ventilated patients resistance and compliance were calculated from airway pressure, gas flow, and tidal volume. In 9 patients with asthma or COPD the course of resistance and compliance was recorded intraoperatively. An increase in resistance of up to 117% of the initial volume occurred during spontaneous respiration, and was caused by a decrease in tidal volume. During mechanical ventilation with constant tidal volume, no definite changes in resistance or compliance were seen with increasing isoflurane concentrations. In the patients with elevated airway resistance there were only minor, statistically non-significant changes in resistance and compliance. The results show that the effects of isoflurane on respiratory mechanics do not differ from those of halothane or enflurane. Therefore, isoflurane may be considered appropriate for use in patients with impaired airway resistance.