The high-resolution computed tomographic (CT) manifestations of Japanese summer-type hypersensitivity pneumonitis in eight patients were correlated with those of their chest radiography and with their histological findings. Histologically proven small granulomas were clearly demonstrated on CT as miliary opacities in five patients whose chest radiographic findings were normal. In two patients, these abnormalities were observed both by CT and chest radiography. High-resolution CT provides detailed morphological information about the lung parenchyma; it is an excellent means of following the clinical courses of hypersensitivity pneumonitis. CT is particularly useful for detecting persistent granulomas when symptoms subside and chest radiography becomes normal or nearly so.