In a prospective study 90 patients with haematologic malignancies (57 acute leukaemias, 6 Hodgkin's Diseases, 15 Non-Hodgkin Lymphomas, 12 other diseases), with fever exceeding 38.4 degrees C and newly developed pulmonary infiltrates underwent bronchoscopy obtaining bronchoalveolar lavage, bronchial washings and protected brush specimen (n = 71). Pneumonias due to gram-negative bacteria (n = 38) and fungi (n = 34) were most frequent. Bronchoscopic specimens yielded 226 isolates (2 different organisms/bronchoscopy on average). 112 organisms were finally regarded as causing pneumonia. Sensitivity of bronchoscopy in diagnosing infectious episodes was 66%, but only 4 out of 13 non-infectious pulmonary infiltrates could be identified. Bronchoscopy was most effective in the diagnosis of pneumocystis carinii and herpes virus pneumonia, whereas sensitivity and specificity of detecting fungal and bacterial pneumonia were low. Empirical antimicrobial therapy was verified by evaluation of bronchoscopic samples in 25 out of 90 cases. Empirical therapy was successfully changed according to the results of invasive samplings in 34 out of 90 cases. Early identification of causative pathogens had a significant impact on survival.