To assess the sensitivity and negative predictive value of sputum examination for Pneumocystis carinii in HIV-positive patients, follow-ups were performed in HIV-infected patients who had been investigated for the presence of Pneumocystis carinii (376 examinations of sputum and 71 bronchoalveolar lavages). Pneumocystis carinii pneumonia was diagnosed 65 times in 64 patients (57 male and 7 female, median age 35 [23-67]years). In 52% of the cases (n = 34) the pathogen was identified in the sputum, in 48% (n = 31) by means of the bronchoalveolar lavage. Of 342 negative findings in sputum examination, five were definitely false negative since the subsequent lavage yielded pneumocysts. In nine further cases Pneumocystis carinii pneumonia could not be excluded because of the course of the disease. In patients from whom sputum samples were available the sensitivity of pathogen identification was at least 70.8% and the negative predictive value at least 95.9%. Since preceding prophylaxis did not render identification in the sputum more difficult, examination of spontaneous or provoked sputum is indicated as a first hand measure in all patients infected with HIV in whom Pneumocystis carinii pneumonia is suspected.