BACKGROUND Based on the changing disease pattern of human immunodeficiency virus (HIV) associated pulmonary complications we conducted a prospective study in order to compare the value of laboratory tests in patients with Pneumocystis (P.) carinii pneumonia (PCP) and other pulmonary complications and of different identification methods of P. carinii in bronchoalveolar lavage fluid (BALF) in PCP patients. METHODS In 217 HIV-1-infected patients we evaluated the following parameters: platelets, serum lactat dehydrogenase (LDH), total serum protein (TP), hemoglobin (Hb), and CD4+ and CD8+ T-lymphocyte count. P. carinii was identified in BALF by May Grünwald Giemsa stain (MGG), direct immunofluorescence test (DIFT), and polymerase chain reaction (PCR). We correlated these parameters in patients with a presumptive diagnosis of PCP and compared them with those of patients suffering from other pulmonary complications. RESULTS All patients underwent bronchoscopy. 55 patients (25.3%) had a presumptive diagnosis of PCP. The sensitivity values of MGG stain, DIFT, and PCR differed considerably (79.1%, 56.1%, and 65.9%, respectively), but specificity values did not (99.2%, 97.3%, and 98.2%, respectively) as well as accuracy values (93.8%, 86.2%, and 89.7%, respectively). The mean values of platelets, of LDH, and of total serum protein of PCP patients and those of patients with other pulmonary diseases differed statistically significant as well as the mean values of these parameters of PCP patients and those of patients with bacterial pneumonia. Logistic-regression analysis revealed the number of platelets and the amount of total serum protein as independent, significant prognostic factors. Moreover, each PCP patient had a CD4+ T-lymphocyte count of less than 200 cells/mm3 blood. The CD4/CD8 ratio of PCP patients was statistically significant lower than that of patients with bacterial pneumonia. CONCLUSIONS A detection of P. carinii in BALF is inevitable for a definitive diagnosis of a PCP. The most efficient identification method in this case is the MGG stain. Platelets, total serum protein, and CD4+ T-lymphocyte count should be included into the criteria for the presumptive diagnosis of PCP.