A case of immunoblastic sarcoma in a 56-year-old man is presented. He had no history of predisposing diseases. His clinical condition was typical of a highly aggressive disseminated malignant lymphoma and he presented important heterogenous hypergammaglobulinemia. The patient died 9 months after the onset of the disease, following brief and incomplete response to various chemotherapeutic associations. The importance of cytological and cytochemical studies of lymph node by touch prep is stressed, since this condition could have been misdiagnosed, in our case, with a malignant histiocytosis. The cell proliferation was shown cytochemically to be of B-lymphoid origin, not histiocytic. It was a monomorphic and nearly massive proliferation of large, intensely basophilic, nonphagocytolytic cells; reactions to naphthol-As-D-acetate esterase, acid phosphatase, beta-glucuronidase, and Perl's stain were negative. The relatively few phagocytolytic cells were shown cytochemically to be normal, true histiocytes, not identifiable with the atypical proliferating cells. This was an essential fact in establishing the diagnosis of immunoblastic sarcoma. In light of today's knowledge, the authors believe that immunoblastic sarcoma is a lymphomatous condition which should be distinguished from centroblastic lymphadenopathy. Lastly, they comment on a retropsective study of lymphomas previously catalogued as reticulo-sarcomas, which has shown that the majority of cases were centroblastic lymphomas and some were immunoblastic sarcomas.