Clinical, physiological and histological investigations on lung involvement are reported in 33 rheumatoid patients. The clinico-pathological patterns of rheumatoid lung disease observed in 19/27 non-smoking female patients were characterized radiologically by diffuse interstitial opacities, functionally by V/Q inequality and microscopically by peribronchiolar and/or alveolar fibrosis. Emphasis is placed on the ventilation/perfusion relationship as well as histopathological studies for a more accurate diagnosis of lung disease in RA. Patients with pulmonary nodules and pleural opacities were also observed. A possible immunological aetiology is suggested on the basis of the simultaneous finding of IgG, complement and fibrinogen in the pulmonary tissue.