To confirm the ability of pulmonary (lung) function testing (PFT) to detect Pneumocystis carinii pneumonia (PCP) before the development of clinical symptoms and to determine if pentamidine (PEN), a drug used in treating PCP, can be given effectively as an aerosol (inhaled mist). Other goals include the measurement of the actual amount of PEN that reaches the lung, and to determine if close clinical observation is safer and as effective as drug therapy for the prevention of subsequent episodes of PCP.