Pneumocystis carinii organisms cluster in alveolar casts. A method for concentrating alveolar casts in sputum specimens might improve the diagnostic usefulness of sputum studies in possible cases of pulmonary infection with Pneumocystis carinii. The use of sputum induction to detect Pneumocystis carinii was studied in 40 consecutive patients with known or suspected human immunodeficiency virus infection having bronchoscopy for evaluation of pulmonary disease. Sputum produced by deep coughing induced by 3% saline via a jet nebulizer was liquefied with dithiothreitol, and cells were sedimented and stained. Liquefaction of mucus allowed concentration of alveolar casts where P. carinii cysts were readily identified. A total of 28 patients were found to have P. carinii, which was detected in concentrated sputum in 21 of 38 patients, unconcentrated sputum in 10 of 38 patients, and by bronchoscopy in 25 of 37 patients. Sensitivity of concentrated sputum compared with bronchoscopy was 78% (95% CI, 58 to 90), with a negative predictive value of 71% (95% CI, 47 to 87). Values for unconcentrated sputum were 43% (95% CI, 25 to 63) and 48% (95% CI, 30 to 67), respectively. Liquefaction of adequately induced sputum facilitates processing and interpretation and increases yield for P. carinii, eliminating the need for invasive procedures in most patients.