Erdheim-Chester disease is a rare non-Langerhans 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-alpha 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-alpha at a starting dose of 3 to 6 x 10(6) units, which was later reduced, during maintenance, to 1 x 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.