106 women and 38 men suffering from definite or classical rheumatoid arthritis underwent a pulmonary function test. The results were correlated to the duration and various clinical activity parameters of the disease. Pulmonary diffusion capacity was found to be reduced compared with the predicted values (p less than 0.05), irrespective of the duration or activity of the disease. The findings suggest that inflammatory vascular changes in the lungs usually occur in rheumatoid arthritis even in cases where other extraarticular manifestations can usually not be demonstrated.