Fibrosing alveolitis was studied by high definition, narrow section computed tomography (CT) in 50 patients, 30 of whom were suffering from systemic sclerosis and 20 from cryptogenic fibrosing alveolitis (CFA). Apart from changes in the mid and lower zones, abnormalities were also observed anteriorly in the upper lobes and in the pleura. Scanning in the prone position was important in seeking evidence of minimal pathological change. In systemic sclerosis CT was 24% more accurate than high kilovoltage radiography in demonstrating minimal evidence of fibrosing alveolitis; this had a characteristic distribution as a peripheral crescent of high attenuation in the lower lobes before any changes were visible on standard radiographs. The CT appearances in CFA were also characteristic, particularly when the disease appeared to be early or limited. We believe that high definition CT may be of value in suggesting the diagnosis of CFA in patients with non-specific abnormalities on the chest radiograph.