Nebulisation of a 10% solution of Mesna (Mistabron) in 10 postoperatively ventilated patients without preexisting pulmonary disease caused a significant increase in inspiratory resistance. This increase is effectively blocked by addition of a bronchodilator (i.e. Salbutamol) to the aerosol. No significant changes in airway resistance were observed in the 10 patients receiving salbutamol alone or isotonic saline. The expiratory resistance did not change suggesting that only the larger airways are involved in the constrictive effect of the drug. Although no patient showed clinical signs of bronchospasm, it is worth considering that even in patients without history of obstructive pulmonary disease nebulisation of Mesna should be performed in combination with a bronchodilator.