Backgrund-Infectio with Pneumocystis carnni typically results in a pneumonia which histologically is seen to consist of an eosinophilic foamy alveolar exudate associated with a mild plasma cell interstitial infiltrate. Special stains show that cysts of P carnnii lie within the alveolar exudate. Atypical histological appearances may occasionally be seen, including a granulomatous pneumonia and diffuse alveolar damage. In these patients the clinical presentation may be atypical and results ofinvestigations negative unless lung biopsies are performed and tissue obtained for histological examination. Methods-The incidence and mode of presentation of histologically atypical pneumocystis pneumonia was studied in a cohort of HIV-I antibody positive patients. Results-Over a 30 month period 138 patients had pneumocystis pneumonia, of whom eight (6%) had atypical histological appearances which were diagnosed (after negative bronchoalveolar lavage) by open lung biopsy in five, percutaneous biopsy in one, and at post mortem examination in two. Atypical appearances included granulomatous inflammation in four patients, "pneumocystoma" in two (one also had extrapulmonary pneumocystosis), bronchiolitis obliterans organising pneumonia in one patient, diffuse alveolar damage and subpleural cysts in one (who also had intrapulmonary cytomegalovirus infection), and extrapulmonary pneumocystosis in two patients. Conclusions-Various atypical histological appearances may be seen in pneumocystis pneumonia. Lung biopsy (either percutaneous or open) should be considered when bronchoalveolar lavage is repeatedly negative and evidence of P caninii should be sought, by use of special stains, in all lung biopsy material from HIV-I antibody positive patients.