Idiopathic interstitial pneumonias are at present classified as one of four types: usual, nonspecific, acute, or desquamative. The acute form has the worst prognosis, followed by the usual and the nonspecific form; it is in desquamative cases that prognosis is best. At high-resolution CT, usual interstitial pneumonia, the most frequent type, manifests as patchy subpleural areas of ground-glass attenuation, irregular linear opacity, and honeycombing, which the nonspecific type, the second most frequent, appears as subpleural patchy areas of ground-glass attenuation with associated areas of irregular linear opacity. Acute interstitial pneumonia demonstrates extensive bilateral airspace consolidation and patchy or diffuse bilateral areas of ground-glass attenuation in middle and lower lung zones.