Summary: A 1‐year‐old infant girl had a chronic illness characterized by persistent intermittent fever, thrombocytopenia, granulocytopenia, and marked hepatosplenomegaly. Histological findings on liver biopsy 307 days after the onset of her illness disclosed marked mononuclear cell infiltration and moderate fibrosis in the portal and perilobular areas. Interferon‐therapy induced no clinical and laboratory improvement, whereas the patient's clinical and biochemical findings responded to prednisone. Although these findings suggested a possible autoimmune mechanism, there were no significant findings in extensive immunological and serological tests consistent with autoimmune disease. The profile of antibodies to Epstein‐Barr virus (EBV)‐specific antigens in the patient demonstrated that this prolonged atypical illness was the result of persistent EBV infection.