The tolerance of aerosolised amphotericin B as prophylaxis against invasive pulmonary aspergillosis was investigated in 61 granulocytopenic periods in 42 patients treated for a haematologic malignancy. Each patient was to receive amphotericin B in doses escalating to 10 mg three times daily (t.i.d.), but only 20 (48%) patients managed to complete the scheduled regimen. One patient tolerated the full dose initially, but had to discontinue treatment when dyspnea developed as a result of pneumonia and acute respiratory distress. Another 22 patients (52%) experienced side effects, including eight (19%) who reported mild coughing and dyspnea but who tolerated the full dose and three (7%) patients whose dose was reduced to 5 mg t.i.d. Another six (14%) patients could tolerate only 5 mg t.i.d., and five (12%) others stopped treatment because of intolerance. Elderly patients (p < 0.05) and those with a history of chronic pulmonary obstructive disease (p = 0.09) were more likely to develop side effects during inhalation. Twelve (28%) patients developed proven of possible invasive fungal infections, but no correlation was established between infection and the total amount of amphotericin B inhaled. Inhalation of aerosolised amphotericin B is poorly tolerated and does not appear useful in preventing invasive pulmonary aspergillosis in granulocytopenic patients.