Interstitial disease is a recognized form of lung involvement in primary Sjögren's syndrome in which frequency and severity have not yet been established. We studied 20 patients 61.2 +/- 36.4 mo after the first symptoms of the syndrome that fulfilled criteria for both xerostomia and xerophtalmia. Eighteen patients exhibited pulmonary symptoms; nine presented a restrictive functional pattern. The chest roentgenogram disclosed interstitial involvement in nine patients. Gallium-67 lung scan presented hypercaptation in 15 of 19 patients studied. Thirteen patients who submitted to bronchoalveolar lavage presented higher cell counts with increases of lymphocytes and/or polymorphonuclear cells. All patients had abnormal results in at least one of the above. Lung biopsy, undertaken in 12 patients, showed a whole spectrum of interstitial disease, from a follicular bronchiolitis to a lymphoid interstitial pneumonia and finally fibrosis with honeycombing. One patient presented with associated sarcoid granulomas. We treated 11 patients with an azathioprine-based regimen and found a significant improvement in the forced vital capacity (p < 0.05) after at least 6 mo when compared with nontreated patients. We conclude that the frequency of interstitial lung disease is high in Sjögren's syndrome and that an azathioprine-based treatment can favorably change the outcome in this population.