Six patients with severe ankylosing spondylitis have been observed to have a characteristic pulmonary fibrosis. In 5 of the 6 patients the condition began radiologically as spotty irregular opacities in the upper halves of the lungs. There was progressive increase in the size and extent of the opacities together with contraction of lung substance. Cysts, sometimes large, then appeared. At this stage productive cough and repeated h˦moptysis developed. Pathological examination has been limited, but a biopsy of the lung in the initial phase displayed a patchy pneumonia with round cell and fibroblastic infiltration. A lobe from a patient with more advanced disease showed dense pleural and intrapulmonary fibrosis with contraction of lung substance and the development of many cysts. There was no evidence that the fibrosis was due to recurrent acute infection, tuberculosis, fungal infection, X-radiation, sarcoidosis, syphilis or pneumonoconiosis. It resembles the fibrosing inflammation which may involve the aorta in spondylitis ankylopoietica and is possibly part of the pathological process of that condition.