Lymphoma of the rectum is a rare tumor and in most studies is not separated from other lymphomas of the large intestine. We have recently examined four homosexual men with lymphoma presenting in the rectum. Symptoms included rectal bleeding in three, pain on defecation in two, and mucoid rectal discharge in two. Systemic "B" symptoms (ie, fever, night sweats, and/or weight loss), as well as a rectal mass, were present in all four. All were high-grade tumors, with B-cell immunoblastic sarcoma in two and small noncleaved Burkitt-like lymphoma in two. Intracytoplasmic immunoperoxidase staining revealed monoclonal kappa light chains in three tumors, whereas the fourth was nonstaining. Immunologic abnormalities were similar to those reported in patients with acquired immunodeficiency syndrome. Antibodies to human T-cell lymphotropic virus type III were found in the three cases tested, and retrovirus was cultured from lymphomatous tissue in one. Despite multiagent chemotherapy, two patients died within six months of diagnosis and a third has recently suffered relapse within the central nervous system.