We report the results of virological (serological and molecular biological) studies of Epstein-Barr virus (EBV) in patients with Kawasaki disease (KD). Forty-nine (86%) of 57 Kawasaki disease patients and 15 (68%) of 22 patients with recurrent Kawasaki disease had serological evidence of primary Epstein-Barr virus infection during the first month after the onset of their disease based on the results of a sensitive method of detecting antibody to viral capsid antigen (VCA). The serological response to EBV was significantly low and transient. EBV sequences were identified directly in peripheral blood mononuclear cell (PBMC) DNA samples from 23 (56%) of 41 KD patients within 2 weeks after onset by means of the polymerase chain reaction (PCR). EBV sequences were also detected in 10 (83%) of 12 repeatedly tested KD patients within 3 months after onset. In contrast, only 7 (18%) of 40 control DNA samples were PCR-positive. These virological studies indicate that an unusual EBV-cell interaction may occur in KD.