Patients with nonspecific interstitial pneumonia present with clinical features that are similar to other idiopathic interstitial pneumonias. The most common symptoms are exertional dyspnea, cough and fever. Lung function testing usually reveals a restrictive defect with decreased gas exchange. The radiological findings are variable and nonspecific. The most common manifestation is areas of consolidation or hazy increased opacity (ground glass opacities) which may be diffuse but tend to involve mainly the lower lobes.

Histopathology reveals mild-to-moderate, mononuclear (predominantly lymphocytes and few plasma cells), chronic interstitial inflammation with varying degrees of fibrosis. The inflammatory infiltrate predominantly involves the alveolar interstitium. Katzenstein and Fiorelli divided the histological pattern of NSIP into three subgroups based on the degree of inflammation and fibrosis: group 1, primarily interstitial inflammation (cellular pattern); group 2, combination of both inflammation and fibrosis (mixed pattern); and group 3, primarily fibrosis (fibrotic pattern). Type II pneumocyte hyperplasia may be seen in cellular pattern. In fibrosing form, dense or loose fibrosis with temporal homogeneity is seen, and the fibroblastic foci that give the heterogeneous appearance in usual interstitial pneumonia (UIP) are characteristically absent.

Hematoxylin-eosin stains may show altered architecture, but elastin stains typically reveal more preserved lung architecture. The alveolar septal lymphocytic infiltrate is less severe and less extensive as compared with lymphocytic interstitial pneumonitis. Similarly, intraluminal organizing fibrosis may be seen in distal airspaces, but it is not a prominent finding as seen in organizing pneumonia pattern. Granulomas, infections by special stains, intra-alveolar pigmented macrophages as seen in desquamative interstitial pneumonitis, hyaline membranes, and severe acute alveolar wall inflammation are absent or inconspicuous.