Prompted by evidence that Langerhans cell histiocytosis (LCH) is a nonmalignant disorder of immune regulation, we used cyclosporine (12 mg/kg/d orally) to treat three young children with advanced multisystem LCH. All three patients had partial responses to cyclosporine within 2 months of therapy, as evidenced by complete resolution of organ dysfunction and regression of the majority of lesions. Complete responses were attained by adding relatively nontoxic chemotherapy (ie, prednisone and vinblastine). Toxicity from cyclosporine comprised mild and reversible elevations of the serum creatinine and blood urea nitrogen. These results indicate that further evaluation of cyclosporine for the treatment of patients with advanced LCH is warranted.