Diffuse interstitial fibrosis of the lung remains a perplexing problem for both pathologist and clinician. In this study of 16 cases the microscopic findings have been divided into three phases: I, early (acute); II, intermediate (subacute); and III, late (chronic). No single etiologic factor can be incriminated in all cases. Morphologic features of the late (chronic) phase as seen in “idiopathic diffuse pulmonary fibrosis” are identical to those observed in the late phase of pulmonary fibrosis seen in the collagen diseases. “Honey-combing” was present in all cases; cor pulmonale was present in ten. Collagen diseases were present in three patients and two others had rheumatoid symptoms of arthralgia. Radiologically, diffuse reticular and fibronodular infiltrates most prominent in the lower lobes of the lungs were constant findings. Pulmonary function studies demonstrated restrictive pulmonary disease and hypoxia at rest, becoming more severe after exercise.