Erythema nodosum is an immunoallergic epiphenomenon appearing during the course of various conditions; it has been exceptionally observed in association with infectious mononucleosis. A 9-year-old girl with clinical manifestations of infectious mononucleosis developed cutaneous nodules of erythema nodosum over the shins, which cleared after 12 days. Diagnosis was based on the presence of atypical mononuclear cells, detection of antibodies against Epstein-Barr virus, and positive ox red blood cell hemolysis. Coincidence of both diseases is not surprising because infectious mononucleosis may exhibit diverse immunologic reactions. On the other hand, commonest etiologies were excluded in this case.