• We assessed the diagnostic value of 46 open lung biopsies performed on 44 immunocompromised children with acute pulmonary infiltrates. We assessed also how frequently the results of these biopsies resulted in a change of therapy. A histological diagnosis was established for all of our patients. At least one infectious agent was found in 72% of the specimens. Nonspecific interstitial pneumonitis was observed in 24% and two biopsy specimens demonstrated relapsed malignancy. In this series, the empirically chosen preoperative therapy was altered on 30 (65%) of 46 occasions. Specific therapy, not provided before surgery, was initiated in 26% of our patients, and broad-spectrum preoperative therapy was discontinued in 43%. Open lung biopsy is a valuable diagnostic procedure in the immunocompromised host with pulmonary infiltrates.