In 43 women and 3 men suffering from primary Sjögren's syndrome pulmonary function was correlated to various clinical, haematological and serological disease activity parameters. Primary Sjögren's syndrome was defined as the presence of keratoconjunctivitis sicca and xerostomia, in the absence of other well-defined chronic inflammatory connective tissue diseases. Only objective tests were used in the evaluation of the patients . Total lung capacity, residual volume, vital capacity, functional residual capacity and ventilatory capacity were normal. The diffusion capacities measured for CO were significantly reduced compared with the predicted values. This indicates that pulmonary interstitial disease is a common feature of primary Sjögren's syndrome. There was a statistically significant negative correlation between reduced diffusion capacity and previous pneumonia, previous pleurisy, tiredness, dyspnoea, ESR and p-orosomucoid. No correlation was found to certain other clinical disease parameters, the duration of disease or tobacco smoking.