Thirty two patients with ankylosing spondylitis were investigated with a set of pulmonary function tests and the results compared with those for a control population. The patients had no complaints about lung symptoms and their chest radiographs were normal. The main pathological findings were reduced lung volumes, a raised closing volume/vital capacity ratio, and a decreased volumic airway conductance. The lung volume reduction correlated with disease duration, thoracic mobility, and degree of acute phase reaction. The stiff spondylitic thorax probably makes the main contribution to the impairment of lung function in these patients, but the findings in this study also suggest an involvement of the small airways. This type of pulmonary function testing seems valuable even in patients with ankylosing spondylitis without lung symptoms and it might be used as a tool in the staging of the disease, to evaluate treatment and to differentiate from fibrosis.