The frequency of atypical pathologic manifestations of Pneumocystis carinii pneumonia (PCP) were studied in 123 lung biopsy specimens from 76 National Institutes of Health patients with the acquired immune deficiency syndrome. The following atypical features were observed: interstitial (63%) and intraluminal (36%) fibrosis, absence of alveolar exudate (19%), numerous alveolar macro-phages (9%), granulomatous inflammation (5%), hyaline membranes (4%), marked interstitial pneumonitis (3%), parenchymal cavities (2%), interstitial microcalcification (2%), minimal histologic reaction (2%), and vascular invasion with vasculitis (1%). These atypical features are discussed with emphasis on the significance of cavities, vascular invasion, vasculitis, and granulomas. Immuno-histochemical staining with monoclonal antibodies to the 2G2 and 6B8 antigens of P carinii in paraffin-embedded lung biopsy specimens did not indicate any diagnostic advantage over routine methenamine silver stains. This study provides an important reminder that a wide variety of pathologic manifestations may occur in PCP in human immunodeficiency virus-infected patients and that atypical features should be sought in lung biopsies from patients at risk for PCP.