We report a patient with eosinophilic granuloma localized to the left mandible who was subsequently shown to have Erdheim-Chester disease involving the lower extremities, omentum and lung. The diagnosis of eosinophilic granuloma was based on the presence of typical CD1a+ Langerhans' cell granulomas in a biopsy of mandible. The diagnosis of Erdheim-Chester disease was established on the basis of the pattern of radioisotopic uptake by long bones, seen on a technetium bone scan, and the presence of characteristic histopathological features in biopsies of lung and peritoneum. The pathological findings in lung were compatible with the abnormalities observed by tomodensitometry, but strikingly different from those seen in Langerhans' cell granulomatosis. The differences in the histological features of pulmonary involvement seen in the two diseases, and the possible relationship between Langerhans' cell granulomatosis and Erdheim-Chester disease, are discussed.