Nonspecific interstitial pneumonia/fibrosis (NSIP) has recently been individualized within the group of idiopathic interstitial pneumonias mainly based on a pathologic pattern of temporally uniform lesions distinct from usual, desquamative, and acute interstitial pneumonia. We studied 12 consecutive patients with NSIP at lung biopsy done as a diagnostic procedure for idiopathic interstitial lung disease. The patients were six males and six females, aged 52.5 +/- 11.8 yr. In 8 of 12 cases the pathologic lesions consisted of both cellular interstitial inflammation and fibrosis, whereas only cellular inflammation was present in three cases, and fibrosis in one. Dyspnea, cough, inspiratory crackles, and squeaks were the most common symptoms and signs. Six cases were cryptogenic. An associated disorder or a presumed cause was present in the other six patients, including underlying connective tissue disease (n = 3), organic dust exposure (n = 2), and prior acute lung injury (n = 1). Lung function tests found a restrictive ventilatory defect (11/12), impairment of TLCO (11/11), and hypoxemia at rest (7/12). Chest X-ray showed infiltrative opacities in all cases. Computed tomography of the chest in 11 cases mainly showed ground glass opacities (9/11), patchy areas of alveolar consolidation (6/ 11), and thickening of interlobular septas (5/11). All patients were treated with corticosteroids, and seven with immunosuppressive agents. All patients were alive at last follow-up, 50 +/- 40 mo after diagnosis. Ten patients (83%) were clinically improved or stabilized. Thus, despite its heterogeneity, NSIP deserves to be individualized as an original clinicopathologic entity and should be clearly distinguished from usual interstitial pneumonia, especially because of a better prognosis.