Abstract: Juvenile Rheumatoid Arthritis (JRA) is a disease of unknown etiology. A total of 50 patients with JRA who were hospitalized in the Pediatrics Rheumatology Ward of Imam Khomeini Hospital in Tehran during the years 2001-2002, were assessed serologically (IgM and IgG specific viral capsid antigens) for EBV infection and their response to therapy was studied. Minimum age of the patients was at least 6 months and mean age was 60.96 plus/minus 43.46 months. EBV infection was seen in 44 (88%) patients 24 of whom were girls and 20 boys. Ninety two percent of girls and 83% boys were infected with the virus. Ebstein barr virus (EBV) infection was seen in 33 cases, 6 cases, 4 cases and 1 case in the polyarticular, pauciarticular, systemic and spondylitis group, respectively. Fifty four percent of EBV-positive patients with JRA did not respond to the classic therapy. EBV virus is involved in the pathogenesis of JRA and patients with EBV are in greater risk of developing JRA.