A chronic infectious mononucleosis-like illness caused by Epstein–Barr virus (EBV) is called ‘chronic active EBV disease’, which is defined as an EBV-associated lymphoproliferative disease. This lymphoproliferative disease is rare and predominantly occurs in Japanese children. Between 1998 and 2010, seven adult-onset cases (aged 20–45 years, median 39 years) were identified, which initially presented with inflammatory diseases, including hepatitis, interstitial pneumonitis, uveitis, nephritis and hypersensitivity to mosquito bites. They showed an EBV viral load in the peripheral blood and evidence of EBV infection of T or natural killer (NK) cells. Five cases (71.4%) developed EBV-positive T/NK-cell lymphoma/leukaemia at a median of 5 years (range 1–7 years) after the diagnosis. Although L-asparaginase-containing chemotherapy was effective for the lymphomas, only allogeneic haematopoietic cell transplantation eradicated EBV-infected cells. This observation indicates that persistent EBV infection of T or NK cells defines a distinct disease entity, which provides an underlying condition for EBV-positive T/NK-cell lymphoma/leukaemia.