Summary A series of 8 patients with rheumatoid arthritis and diffuse interstitial lung disease has been described. In all but one the joint symptoms preceded the pulmonary changes. There was no correlation between the occurrence of pulmonary changes and the duration or severity of the joint condition, nor were these related to the level of circulating rheumatoid factor. Four patients were given corticosteroids, and in 3 an initial improvement was followed by deterioration. The fourth patient showed no response. From these cases and others reviewed in the literature, it is concluded that the degree of dyspnœa is the best guide to prognosis. The subsequent development of a pulmonary tumour in 2 of the 8 patients is described and discussed. The chest radiographs of 177 patients suffering from rheumatoid arthritis were compared with those of a control group matched for age and sex. There was no significant difference in the frequency of pleuro-pulmonary abnormalities between the two groups, although the higher incidence in males of the diffuse radiological abnormality of the type seen in interstitial lung disease might be shown to be significant in a larger series. There is a need for further investigation of the incidence of pleuro-pulmonary changes in rheumatoid disease. Not until this has been carried out on a far wider scale than has been hitherto attempted can it be established whether such changes are a specific feature of the condition.