The patients with chronic active Epstein-Barr virus infection (CAEBV) in childhood in Japan are described. Among 39 registered cases, 20 patients were males and 19 were females. Unlike the X-linked lymphoproliferative syndrome, there was no hereditary background. The incidence of hypersensitivity to mosquito bites was high (31.3%) as a past history. Most patients exhibited hepatomegaly (92.3%), splenomegaly (87.2%) and fever (84.6%). The incidence of absent anti-EB virus nuclear antigen titres was unexpectedly low (17.1%). Lymphoreticular disorders and cardiovascular diseases were major complications. Twenty-four (61.5%) patients died 6 months to 8 years after the onset, mainly of hepatic failure (eight cases), cardiac failure (five cases), virus-associated haemophagocytic syndrome (three cases) and haematological malignancies (two cases). This study reveals that CAEBV in Japan has several clinical features and should be informative for the pathogenesis of EB virus.