A 4½-year-old boy experienced the acute onset of fever, generalized lymphandenopathy, apparent respiratory obstruction, and hepatosplenomegaly. Laboratory studies were compatible with infectious mononucleosis. In order to rule out the possibility of a fulminant lymphoma and govern the therapy, a cervical lymph node biopsy was performed. Histological examination revealed architectural distortion and engorgement with atypical reticulum cells, some of which were indistinguishable from Reed-Sternberg cells. However, an interpretation of Hodgkin's disease was deferred, and the patient was treated conservatively. Thirteen months later he was asymptomatic, and his abnormal physical findings and laboratory tests returned to normal. This patient is presented, therefore, as an instance of the presence of Reed-Sternberg cells in a disease other than Hodgkin's.