Usual interstitial pneumonitis (UIP) is a chronic pulmonary process with a characteristic peripheral fibrotic pattern on gross pathologic lung sections and CT scans. This condition is often idiopathic, but asbestosis, rheumatoid arthritis, and scleroderma may cause the same peripheral fibrosis in the lungs. UIP is associated with an increased incidence of pulmonary neoplasms. The purpose of this study was to evaluate the size of mediastinal lymph nodes in patients with UIP in whom no evidence was seen of malignancy or current active infection. CT scans of 14 patients (12 with idiopathic pulmonary fibrosis and two with collagen vascular disorders) were assessed for lymph node location (American Thoracic Society mediastinal map) and size. In 13 of 14 patients, nodes measured greater than threshold size values. Nodes as large as 20 x 30 mm were identified in three patients. Nodal sites 10R, 4R, 2R, 5, and 6 were most commonly abnormal. We conclude that increase in the size of mediastinal lymph nodes as shown on chest CT scans is common in patients with UIP, occurs without superimposed infectious or malignant complications, and is thus presumably part of the chronic inflammatory process. Consequently, lymphadenopathy in these patients does not suggest that they have lung cancer also.