HIV-associated malignant lymphomas are a common complication in late HIV infection, and there is a high percentage of gastrointestinal tract involvement. Non-Hodgkin's lymphoma was found in 108 of 2,750 HIV-positive patients (3.9%) in our institution, whereas gastrointestinal manifestation was diagnosed in 48 of 108 patients (44.4%). 44 of these cases were found during endoscopy of the upper and lower gastrointestinal tract (or by laparotomy or laparoscopy in 4 cases). Endoscopy is a reliable procedure for the diagnosis of lymphoma. Unusual manifestations such as oral, esophageal or perianal lesions and multifocal disease were common findings. Life-threatening complications such as gastrointestinal bleeding, perforation, and obstruction occurred in 37.5%. High-grade B-cell lymphomas were found in all cases including mainly lymphoblastic, immunoblastic, centroblastic and Burkitt subtypes. 52% of the patients had disseminated lymphoma with Ann Arbor stage III or IV. Standard chemotherapy with cyclophosphamide, adriamycin, vincristine and prednisone was started in 25 patients and resulted in a mean survival time of 4.8 months. The prognosis of AIDS patients presenting with malignant gastrointestinal lymphoma depends mainly on the presence or absence of previous AIDS-defining diseases, not CD4 cells, lymphoma-associated gastrointestinal complications or the histopathologic lymphoma type at the time of diagnosis.