The purpose of this study was to evaluate the diagnostic yield of induced sputum (IS), assessing the reliability of indirect immunofluorescent stain with monoclonal antibodies (IFMoAb) and methenamine silver (Met-Ag) and analysing factors likely to influence the sensitivity of these techniques. An analysis was prospectively carried out on IS specimens collected from 61 human immunodeficiency virus (HIV)-infected patients during 69 episodes of suspected Pneumocystis carinii pneumonia. Ultrasonic nebulizers with hypertonic 2% saline were used. IFMoAb to P. carinii and Met-Ag were performed after cytocentrifugation of the specimen. Results were compared with those of bronchoalveolar lavage (BAL) with/without transbronchial biopsy (TBB), performed not more than seven days after induction of sputum. P. carinii pneumonia was confirmed in 32 episodes, of which IS was diagnostic in 23. The sensitivity of the staining procedures was 69% for IFMoAb, and 28% for Met-Ag. The three episodes of P. carinii pneumonia in patients on oral chemoprophylaxis yielded negative IS results; in contrast, IS was negative in only 6 of the 29 cases not receiving chemoprophylaxis. IS is a non-aggressive procedure that diagnosed P. carinii pneumonia in 72% of our cases. The yield increased significantly when IFMoAb was used in patients not receiving oral chemoprophylaxis.