BACKGROUND--Bronchoalveolar lavage is considered a safe procedure. When performed in the mechanically ventilated patient, however, potentially harmful effects on respiratory and haemodynamic functions have been reported in which the size of the bronchoscope may play a part. Two different size bronchoscopes (adult and paediatric) were therefore investigated with regard to bronchoalveolar lavage yield and side effects. METHODS--Twenty mechanically ventilated patients underwent bronchoalveolar lavage with both adult and paediatric bronchoscopes in a randomised sequential manner. RESULTS--In a total of 45 pairs of bronchoalveolar lavage procedures no difference was noted between adult and paediatric bronchoscopes with regard to total cell yield, differential cell count, and microbiological results. Peak intratracheal pressure increased with the adult bronchoscope only. Systemic arterial pressures increased more with the adult than with the paediatric bronchoscope. PaO2 decreased with the adult but not with the paediatric bronchoscope. CONCLUSIONS--The paediatric bronchoscope offers a comparable bronchoalveolar lavage yield in mechanically ventilated patients to the adult bronchoscope, while the respiratory and haemodynamic side effects are significantly lower than with the adult size instrument.