Erdheim-Chester disease is a rare, non-Langerhans systemic histiocytosis characterized by bilateral sclerosis of the metaphyseal regions of the long bones and infiltration in other organs. The histopathologic hallmark is defined by a mononuclear infiltrate of foamy histiocytes and rare pathognomonic Touton giant cells with extensive fibrosis. This condition is exceptional in children. We report here a case of Erdheim-Chester disease in a 10-year-old girl with retroperitoneal infiltration and bone involvement, for whom the diagnosis was only established after a 3-year course with multiple biopsies. It is also the first pediatric case successfully treated with interferon-α suggesting that interferon-α can be a safe and efficient first-line therapy for this disease in children.