Erdheim-Chester disease is a rare nonLangerhans histiocytosis with multisystem involvement. To date, there is no standard treatment for this disorder, and more than half of the patients succumb within 3 years. Because interferon-␣ promotes the terminal differentiation of histiocytes and dendritic cells, we hypothesized that this molecule would be a useful therapy for Erdheim-Chester disease. We therefore treated 3 patients with advanced disease with interferon-␣ at a starting dose of 3 to 6 ؋ 10 6 units, which was later reduced, during maintenance, to 1 ؋ 10 6 units subcutaneous 3 times per week. Marked improvement was noted in all patients, with substantial retro-orbital disease regression within 1 month. Improvement in bone lesions, pain, diabetes insipidus, and other manifestations was gradual over many months. Responses were durable (3؉ to 4.5؉ years). Our observations suggest that this well-tolerated therapy has a significant effect on the course and outcome of Erdheim-Chester disease. (Blood. 2005;106:2992-2994)