Nonspecific interstitial pneumonitis with fibrosis has been individualized within the group of idiopathic diffuse interstitial pneumonias by pathological criteria. It is differentiated from usual interstitial pneumonitis by the temporal uniformity of the lesions, a prominent inflammatory interstitial infiltration, and the absence of honeycombing. Clinical and functional symptoms are those of diffuse interstitial pneumonitis. An etiology may be found in about half the cases, including connective tissue disease, exposure to organic antigens, or recent acute lung injury. Computed tomography of the chest shows bilateral ground glass opacities, and alveolar opacities with a peribronchiolar or patchy distribution. Prognosis is rather good, since a majority of patients improve when treated with corticosteroids or with an association of corticosteroids and immunosuppressive drugs. These etiologic and prognostic features justify the individualization of nonspecific interstitial pneumonitis with fibrosis as a distinct clinicopathological entity.