A two year old boy exhibited not only clinical manifestations which suggested a recurrence of Kawasaki disease (KD) but also evidence of a primary infection by Epstein-Barr virus (EBV) including tonsillitis, splenomegaly and atypical lymphocytosis in the peripheral blood. An inverted CD4/CD8 ratio in lymphocyte subsets suggested the presence of infectious mononucleosis (IM). Epstein-Barr virus titers (viral capsid antigen-immunoglobulin G 1:20; Epstein-Barr virus-associated nuclear antigen < 1:10) showed an acute EBV infection and the presence of EBV genome in the blood was determined by the polymerase chain reaction technique. In Japan, the peak incidence of KD and IM is in children under 4 years of age. From the investigation of EBV titers, it has been reported that some patients with KD develop an associated, unusual primary EBV infection. Kawasaki disease concurrent with a primary EBV infection as in this case, suggests the possibility of an etiologic agent related to the KD rather than to the EBV infection itself.