BACKGROUND: Nonspecific interstitial pneumonia (NSIP) has been reported recently to show much better response to medical treatment and better prognosis compared with idiopathic UIP. However, clinical characteristics of idiopathic NSIP discriminating from UIP have not been defined clearly. METHOD: Among 120 patients with biopsy-proven diffuse interstitial lung diseases between July 1996 and March 2000 at Samsung Medical Center, 18 patients with idiopathic NSIP were included in this study. Retrospective chart review and radiographic analysis were performed. RESULTS: 1) At diagnosis, 17 patients were female and average age was 55.2 +/-8.4 years (44~73 years). The average duration from development of respiratory symptom to surgical lung biopsy was 9.9+/-17.1 months. Increase in bronchoalveolar lavage fluid lymphocytes (23.0 +/-13.1%) was noted. On HRCT, ground glass and irregular linear opacity were seen but honeycombing was absent in all patients. 2) Corticosteroids were initially given to 13 patients of whom medication was stopped in 3 patients due to severe side effects. Further medical therapy was impossible in 1 patient who experienced streroid-induced psychosis. Herpes zoster (n=3), tuberculosis (n=1), avascu lar necrosis of hip (n=1), cataract (n=2) and diabetes mellitus (n=1) developed during prolonged corticosteroid administration. Of 7 patients receiving oral cyclophosphamide therapy, hemorrhagic cystitis hindered one patient from continuous medication. 3) After medical treatment, 14 of 17 patients improved and 3 patients remained stable (mean w-up ; 24.1+/-11.2 months). FVC increased by 20.2 +/-11.2% of predicted value and the extent of ground glass opacity on HRCT decreased significantly (15.7+/-14.7%). 4) Of 14 patients who had stopped medication, 5 showed recurrence of NSIP and 2 aggravated during steroid tapering. All patients with recurrence showed deterioration within one year after completion of initial treatment. CONCLUSION: Since idiopathic NSIP has unique clinical profiles and shows a good prognosis, differential diagnosis from UIP and aggressive medical treatment are needed.