We retrospectively studied the outcome of percutaneous needle biopsy and intralesional injection of a corticosteroid (methylprednisolone) in thirty-nine patients who had localized Langerhans-cell histiocytosis (eosinophilic granuloma of bone). All thirty-nine patients had a solitary symptomatic lesion at presentation; a second lesion developed in two patients, two and four months after the first lesion was diagnosed. Therefore, there were forty-one lesions in thirty-nine patients. Fine-needle aspiration with or without core-needle biopsy was performed for all forty-one lesions, and the diagnosis of Langerhans-cell histiocytosis was established for thirty-seven (90 per cent). A corticosteroid was injected into thirty-five lesions. Twenty-nine received the injection at the time of the fine-needle aspiration on the basis of the cytological findings in the aspirate. Six patients who had a solitary lesion had a two-stage procedure because the injection was delayed until the diagnosis was confirmed with histological evaluation of specimens obtained by core-needle biopsy. Thirty-four (97 per cent) of the thirty-five lesions healed. The clinical symptoms associated with thirty-one lesions resolved within two weeks after a single injection of the corticosteroid. There were no complications associated with either the biopsy or the injection. At a median of ninety months (range, twenty-four to 199 months), no patient had recurrence of symptoms or of radiographic evidence of the lesion. All patients who had been managed with an intralesional injection of the corticosteroid had full range of motion of the affected extremity and had resumed unlimited activities. Although the mechanism of action of intralesional injection of a corticosteroid has not been defined, use of percutaneous needle biopsy to diagnose localized Langerhans-cell histiocytosis and treatment with intralesional administration of methylprednisolone relieved pain expeditiously, enabled the patient to avoid an operative procedure, and resulted in osseous healing. The specific role of corticosteroid therapy remains to be determined by prospective, randomized studies.