Background: Pulmonary complications in immunocompromised patients are often fatal. Empirical treatment is usually applied based on the clinical and radiological findings because of the risk of the aggressive diagnostic procedures such as open lung biopsy. However, recent advancements in operative procedures and perioperative management has decreased the procedure-related risks. We have prospectively analyzed the risks and benefits of the early application of open lung biopsy in such patients. Material and Method: Forty-two consecutive immunocompromised patients with critical pulmonary complications were included from June, 1996 to December, 1999. The definition of the immunocompromised is as those with chemotherapy and/or other modality for hematologic disorders, with usage of immunosuppressive drug after transplantation, with usage of steroid for more than 1 month, and with primary immunodeficiency disorders. The indication of open lung biopsy was those with no significant improvement after a week of aggressive application of empirical treatment or with rapidly aggressive process. The underlying disease included hematologic disorder(31 patients), post-transplantation(3 patients), chemotherapy for solid tumor(2 patients), and others(6 patients). Operations were done through thoracotomy(conventional or mini-) or VATS.