Pulmonary involvement due to disseminated non Hodgkin lymphoma (LNH), is an unusual cause of lung disease in AIDS patients. We report a 38 years old male patient, with advanced AIDS, who, in the course of three weeks, developed cough, dyspnea and fever. The chest X ray film showed diffuse thickening of the peribronchovascular connective tissue with possible mediastinal lymph node enlargement. The evolution was unfavorable with hypoxemia, severe anemia, liver damage and elevated levels of lactic dehydrogenase. The presumptive initial diagnoses were Pneumocystis carinii pneumonia, pulmonary tuberculosis with hematogenous dissemination and Kaposi sarcoma. Definitive diagnosis was made through a transbronchial biopsy performed the day before his death. The pathological and inmunohistochemical report demonstrated a highly aggressive lymphoma (lymphoblastic, B precursor). This finding was confirmed by autopsy that revealed multiple organ involvement (Rev Méd Chile 2001; 129: 1315-9).