Pulmonary toxicity with acute dyspnea occurred during infusion of a liposomal amphotericin B preparation (AmBisome) in two adult leukemic patients. The preparation was administered as a one hour infusion at a dose of 3 mg/kg body weight. Within 15 min after starting the infusion, both patients experienced sudden onset of dyspnea and chest tightness. Physical examination showed the patients to be anxious and restless with tachycardia and orthopnea but without other cardiopulmonary findings. No elevation of body temperature, rigors or chills were recorded. Symptoms disappeared within minutes after discontinuing the infusion. At present, the pathophysiologic mechanisms underlying these side effects are unknown.