Pneumocystis carinii pneumonia is an infectious disease which is highly prevalent in the group of immunosuppressed patients, particularly with hematologic tumors as lymphomas and acquired immune deficiency syndrome(AIDS), severe malnutrition, organ transplantations, high dose corticosteroid therapy. Some cases of Pneumocystis carinii pneumonia in infants with primary immune deficiency were already reported. The authors present a case of Pneumocystis carinii pneumonia developed in an infant who suffered from 10 days of poor feeding and failure to thrive and not included in the risk groups listed above. He had bilateral interstitial infiltrations on the chest radiography, diagnosed as Pneumocystis carinii pneumonia after Gomori-methenamine silver staining of his sputum that was taken through tracheal intubation. He improved after administering Trimethoprim-sulfamethoxazole for 14 days.