To determine the role of open lung biopsy in immunocompetent patients with community-acquired pneumonia who require hospitalization. A group of 1,118 patients with severe community-acquired pneumonia that required hospitalization were enrolled in the study. Of the patients, 26 underwent open lung biopsy. Another 18 of these patients were immunocompromised and were excluded from this segment of the study. Tertiary care 800-bed hospital from November 1981 to May 1989. Progressive diffuse pulmonary infiltrates and negative conventional cultures were the indications for biopsy in most of these patients. Eighteen (69 percent) were immunocompromised. The eight immunocompetent patients underwent a retrospective review of their course in hospital. Three patients died. The diagnostic yield from open lung biopsy was 25 percent. A specific histologic diagnosis was made in one patient--lipoid pneumonia. The pulmonary histologic finding were nonspecific in the remaining patients, but in four, in combination with the clinical data, gave useful information and resulted in therapy change. Culture of a pulmonary tissue yielded cytomegalovirus in one other patient. Serologic testing had a low yield in this group with three patients having a positive result. Open lung biopsy is rarely necessary in immunocompetent patients with community-acquired pneumonia. In a small group of patients where it is necessary, however, both positive and negative results are important in directing therapy.