Patients ventilated in the pressure support mode must generate a negative airway pressure before the ventilator will deliver a breath. Inserting a continuous-flow nebulizer between the patient and the sensor in the ventilator makes it more difficult for the patient to generate this negative pressure. We observed two mechanically ventilated patients who were unable to initiate ventilator breaths in the pressure support mode while bronchodilators were being administered through a continuous-flow nebulizer. In neither case did ventilator alarms sound. Using a lung model, we found that when the nebulizer flow rate exceeded the mean inspiratory flow rate of the test lung, the negative pressure necessary to trigger the pressure support ventilator could not be generated. Critical care providers need to be aware of this potential complication, since it may lead to serious underventilation of their patients.