We report a case of infectious mononucleosis progressing to fatal immunoblastic lymphoma. The patient, a 44-year-old man who may have had an immunoregulatory defect, failed to have an appropriate T-cell response to his Epstein-Barr (EB) viral infection. His active EB viral infection was manifest by seroconversion of IgM-viral capsid antibody and a greater than four fold rise in IgG-viral capsid antibody. Also, he transmitted his EB viral infection to his wife who became ill 1 month after his death. Clinically the patient's illness was characterized by waxing and waning lymphadenopathy, persistent fever, diarrhea (similar to that associated with cholera), a coagulopathy, and gastrointestinal bleeding. The patient had pathologic findings of a diffuse immunoblastic lymphoma involving lymph nodes, small bowel, liver, pancreas, kidneys, lungs, and bone marrow. Immunologic cell markers showed the tumor to be polyclonal.