Twenty-seven patients with eosinophilic granuloma (EG) of bone seen at our institution between 1979 and 1991 underwent fine-needle aspiration (FNA) with or without concurrent Tru-Cut biopsy. The 16 males and 11 females ranged in age from 2 1/2 to 61 years (median, 10 yr). Twenty-four patients had monostotic lesions. The clinicoradiologic differential diagnosis included osteomyelitis and Ewing's sarcoma (young patients) and primary and metastatic malignancies (older patients). Twenty-four of 28 FNAs (one patient had two FNAs) were diagnostic of EG, and 10 cases were diagnosed by FNA alone. Smears in these cases showed histiocytes, often with grooved or infolded nuclei, and abundant eosinophils. Multinucleated giant cells, foamy histiocytes, neutrophils, lymphocytes, and plasma cells were present in variable numbers. Four FNAs were misdiagnosed: two as osteomyelitis where smears contained abundant neutrophils, sparse eosinophils, and histiocytes misinterpreted as foamy histiocytes, and two as metastatic carcinoma (in adults) where histiocytes in a scant specimen (one case) and skin appendiceal structures without lesional tissue (one case) were misinterpreted. These cases were correctly diagnosed on repeat FNA (one case), Tru-Cut (two cases), or excisional biopsy (one case); however, three cases diagnosed by FNA had nondiagnostic concurrent Tru-Cut biopsies. Treatment consisted of intralesional injection of 125 mg of methylprednisolone (22 cases). Progressive or complete healing of all lesions occurred. FNA is a rapid and useful technique for the immediate diagnosis of EG that allows concurrent institution of therapy.