A 47-year-old woman was admitted to our hospital because of dry cough, fever, and subacute, progressive dyspnea. Chest radiography and computed tomography showed ground glass opacities in the lower lung fields. We suspected pneumonia caused by atypical pathogens and administered antibiotics, but they had no effect at all. Histopathologic findings from a transbronchial lung biopsy (TBLB) included intensive infiltration of mononuclear cells and edema on the alveolar wall with no evidence of fibrosis, fibroblasts, hyaline membrane, or granuloma. On the basis of these findings, we suspected interstitial pneumonia, but a surgical lung biopsy was not possible because the patient would not give her consent. After TBLB, corticosteroid was administered repetitively, but dyspnea was deteriorating as the ground glass opacities became more widespread, and tractional bronchiectasis appeared throughout the lung fields. Therefore, we decided to administer cyclophosphamide (CPA). This was very effective: all of her symptoms improved and the ground glass opacities and tractional bronchiectasis disappeared. Though we tapered and then discontinued corticosteroids a few months after CPA, there was no recurrence whatever. No signs suggesting the association of collagen vascular diseases were detected. The effectiveness of CPA in interstitial pneumonia associated with collagen vascular disease is occasionally reported, but the effect on idiopathic interstitial pneumonia, especially in acute and subacute progressive cases, is rarely reported. We think this is an interesting case to consider the availability of CPA in idiopathic interstitial pneumonia with subacute progression.