A 63-year-old female with acure exacerbution of idiopathic interstitial pneumonia (IIP) showed high levels of elastolytic activity in bronchoalveolar lavage fluid (BALF). She was admitted to our hospital with progressive dyspnea. Arterial blood gas analysis and pulmonary function tests were abnormal. Examination of the bronchoalveolar lavage fluid revealed an increased numbers of neutrophils. Despite the administration of corticosteroids, the patient died of respiratory failure. A high level of elastolytic activity was present in the BALF. Western immunoblot analysis, using an anti-alpha 1-protease inhibitor (PI) antibody, revealed a truncated alpha 1-PI in the BALF. These findings suggest that an imbalance between protease and PI in the lower respiratory tract contribute to lung tissue damage in patients with IIP.