Sixteen patients are assigned into one of the following groups. Group 1 (four patients) receives intravenous pentamidine as a one-time dose, infused over 2 hours. Group 2a (six patients) receives aerosolized pentamidine via face mask. Group 2b (six patients) receives aerosolized pentamidine 2 times. Group 2b will be studied only if initial dose is well tolerated. Small amounts (1 - 2 cubic centimeters) of blood is taken from all groups at 40 minutes, and 2, 3, 7, 14, and 24 hours from the beginning of pentamidine treatment and at the same time as the lung biopsy or bronchial alveolar lavage. Patients are given routine TMP / SMX (or whatever medications are considered appropriate by the patient's primary physician for medical management) dosing 1 - 2 hours after pentamidine is given. Bronchial alveolar lavage fluid or lung tissue from biopsy will be obtained between 2 - 48 hours after initiation of pentamidine treatment (optionally 10 - 24 hours post dose).