Plasma levels of the atrial natriuretic factor were prospectively measured in 15 patients suffering from connective tissue disease with alveolar-interstitial lung lesions. The mean level was significantly higher in these patients than in 12 patients without known lung lesions (74.8 +/- 20.7 pg/ml versus 30.8 +/- 12 pg/ml; P less than 0.01) and in 18 healthy subjects (25.4 +/- 12.6 pg/ml). There was no correlation between these levels and lung function tests. Three patients with isolated lymphocytosis in the alveolar lavage fluid had a high level of atrial natriuretic factor.