Pneumocystis carinii pneumonia (PCP) has become a major cause of morbidity and mortality due to infectious diseases, largely as a result of the acquired immunodeficiency syndrome (AIDS) epidemic. Since the mortality from recurrent infection is between 40% and 60%, early diagnosis and therapy are the keys to survival. The role of technetium 99m diethylene triamine penta-acetate (DTPA) aerosol pulmonary clearance was studied in 81 patients with AIDS. The mathematical technique of curve stripping was found to be the optimal method of analysis and to provide an overall sensitivity of 94% for the detection of interstitial pneumonitis. The procedure was superior to standard pathology parameters and radiography and more convenient than gallium 67 scintigraphy.