In 16 children with multisystem Langerhans cell histiocytosis (mean age 22 months, range 5 to 36 months) severe symptomatic skin involvement was treated with topical nitrogen mustard (mechlorethamine hydrochloride). In each case, rapid clinical improvement occurred within 10 days; subsequent complete healing was observed in 14 children, and partial healing in 2 others in whom treatment was a component of palliative care. Mean duration of treatment was 3.5 months (range 2 to 6 months). Systemic treatment was averted in 11 patients because response to topical therapy was so favorable, but bone marrow or respiratory failure led to a fatal outcome in 5 other patients. Adverse effects were minimal. One patient developed contact allergy to topical nitrogen mustard after 2 years of intermittent therapy, but was successfully desensitized and was then able to continue treatment. We conclude that the topical application of nitrogen mustard is an effective treatment for cutaneous Langerhans cell histiocytosis. Although adverse effects were minimal in the short term, there remains concern about the possibility of long-term cutaneous carcinogenicity.