OBJECTIVE: Erdheim-Chester disease (ECD) is a rare form of non-Langerhans' cell histiocytosis that may present with pulmonary involvement. We undertook the current study to evaluate the characteristic features of pulmonary involvement in ECD, in the largest single-center series of patients reported to date. METHODS: We performed a retrospective study of the characteristics of 34 consecutive patients with biopsy-proven ECD who were referred to the internal medicine department of Pitie-Salpetriere Hospital between 1981 and November 2008. RESULTS: Data were obtained from 23 men and 11 women. The median age at the time of diagnosis was 53.7 years (range 16-73 years), and the median followup was 3.5 years (1.4-5.3 years). Eight patients (24%) had pulmonary symptoms. High-resolution computed tomography (HRCT) scans of the chest revealed involvement of lung parenchyma in 18 patients (53%) and of the pleura in 14 patients (41%). Bronchoalveolar lavage fluid analysis revealed the presence of an opalescent aspirate in all the patients studied. Treatment with corticosteroids and/or interferon-alpha (IFNalpha) resulted in a marked improvement of the pulmonary lesions in only a single patient. Comparison of the survival between patients with and those without pulmonary involvement yielded no significant difference between the groups (P = 0.82). CONCLUSION: Pulmonary involvement in ECD has been overlooked in previous studies. HRCT reveals typical lesions in most patients. There is no clear response of these lesions to corticosteroids and IFNalpha. The overall prognosis of the disease is poor, but pulmonary involvement does not appear to be a major prognostic factor in ECD.