Pulmonary function was measured in 24 sarcoidosis patients, 17 nonsmokers and 7 smokers. 12 (4 smokers) had evidence of small airway disease and 6 patients (3 smokers) had evidence of large airway disease. A significantly greater proportion of smokers had an increased closing volume, and closing volume appears to be the most sensitive test for small airway disease in sarcoidosis. However, 3 patients with normal closing volumes had evidence of small airway disease by forced expiratory flow rates or frequency dependence of compliance. We conclude that there is a high incidence of small airway disease in patients with sarcoidosis. There also appears to be a synergism between sarcoidosis and smoking that leads to a significant degree of hyperinflation of the lung.