An elderly woman is described with infectious mononucleosis in whom cervical node biopsy was interpreted as showing immunoblastic lymphoma. Concomitant reactive lymphocytosis, Epstein-Barr virus serologic results consistent with an acute infection, and demonstration of polyclonal B cell infiltration of other tissues argued against intervention. Defective in vitro T cell responses were demonstrated during the acute phase of Epstein-Barr virus infection. Infectious mononucleosis has rarely been reported as mimicking a non-Hodgkin's lymphoma. At 18 months, our patient's course has been typical for infectious mononucleosis with no evidence of disseminated malignancy.