RESUMEN
Pneumocystis carinii is the most common cause of diffuse pulmonary infiltrates in the immunocompromised patients. Microscopically, Pneumocystis carinii pneumonia(PCP) shows characteristic frothy intraalveolar exudate and interstitial lymphocytic and plasma cell infiltrate. However, sometimes the only histologic finding of PCP on routine hematoxylin-eosin stain is that of diffuse alveolar damage(DAD), when we can miss the diagnosis without aid of special stains. We report a case of Pneumocystis carinii pneumonia presenting as DAD in a 50-year old man after chemotherapy due to malignant lymphoma. Open lung biopsy specimen reveals the early stage of DAD without any characteristic findings, such as foamy exudate. However many cysts of Pneumocystis carinii were found on Gomori's methenamine silver(GMS) stain. Therefore, GMS stain should be routinely performed on all biopsy specimens obtained from immunocompromised patients.