Idiopathic interstitial pneumonias are currently classified into four categories: usual interstitial pneumonia, nonspecific interstitial pneumonia with fibrosis, acute interstitial pneumonia and desquamative interstitial pneumonia. The fibrotic process in interstitial pneumonias appears to result from a complex interaction between fibroblasts, other lung parenchymal cells and macrophages. The complex relationship between the local release of growth-promoting cytokines by alveolar macrophages and resident fibroblasts represents a necessary step for fibrosis or remodeling after lung injury. Injury to the epithelium and basement membranes is likely necessary for the fibrotic process to occur. Usual interstitial pneumonia, most frequent among interstitial pneumonias and has a poor prognosis, appears on high-resolution CT as patchy subpleural areas of ground-glass attenuation, irregular linear opacity, and honeycombing. Nonspecific interstitial pneumonia with fibrosis, the second most frequent and has a better prognosis than usual interstitial pneumonia, appears as subpleural patchy areas of ground-glass attenuation with associated areas of irregular linear opacity on CT. Acute interstitial pneumonia with high mortality rate presents as extensive bilateral airspace consolidation and patchy or diffuse bilateral areas of ground-glass attenuation. Desquamative interstitial pneumonia with good prognosis presents as patchy subpleural areas of ground-glass attenuation in middle and lower lung zones.