Acute interstitial pneumonia is a specific disease that is different from the chronic interstitial pneumonias clinically and pathologically. Acute interstitial pneumonia is charcterized by a sudden onset with dyspnea, cough, and often fever, followed by respiratory failure. The mortality rate is high and the course is also rapid. Histologically, acute interstitial pneumonia can usually be distinguished from the chronic interstitial pneumonias even without knowledge of the clinical history. That is, the fibrosis in acute interstitial pneumonia is characterized by numerous fibroblasts within an edematous-appearing stroma but with relatively little collagen deposition. We experienced a case of acute interstitial pneumonia in 52-year old female who showed clinical, radiological and histologic features of acute interstitial pneumonia without any identified etiologic agent.