Nonspecific interstitial pneumonia/fibrosis (NSIP) was first described by Katzenstein and Fiorelli in 1994 (Am J Surg Pathol 18: 136-147). Many reports have described that corticosteroids are effective for NSIP.Wedescribe a case of group II idiopathic NSIP in whomcyclophosphamide was administered since the initial response to corticosteroids had been insufficient. Lung biopsy was performed by video-assisted thoracoscopic surgery and NSIP was diagnosed pathologically, clinically and radiologically. Although the initial response to corticosteroids was insufficient, interstitial infiltrates on chest computed tomography improved dramatically after adding intravenous cyclophosphamide followed by oral cyclophosphamide. This case demonstrates that the addition of cyclophosphamide to corticosteroids might be a useful treatment for patients with NSIP. (Internal Medicine 41 : 867-870, 2002)