One hundred individuals who had undergone both high resolution computed tomography (HRCT) and chest radiography were studied to determine the accuracy of each technique in establishing the diagnosis of diffuse lung disease. The group consisted of 86 patients with a diagnosis of a chronic diffuse infiltrative lung disease and 14 normal subjects. Two independent observers assessed the HRCT examinations and chest radiographs and recorded the three most likely diagnoses. Overall a confident diagnosis was reached more often with HRCT (49%) than with chest radiography (41%). The diagnoses were correct in 82% of HRCT examinations and 69% of chest radiographs. Diagnoses made on HRCT, irrespective of the degree of certainty, were accurate more often than diagnoses made on chest radiography (56% and 47% respectively). Of the patients thought to have a normal chest radiograph, 42% had diffuse infiltrative lung disease (DILD). Of the patients thought to be normal on HRCT, 18% had DILD. Conversely, normal subjects were correctly identified as such in 82% of chest radiographs and in 96% of HRCT examinations. This study emphasizes the important role of CT in helping to confirm or refute the presence of abnormality when the chest radiograph is normal or questionably abnormal, and underlines the superior diagnostic accuracy of HRCT compared with conventional chest radiography in DILD.