We treated three patients with idiopathic pulmonary fibrosis who had an acute clinical exacerbation. We analyzed their clinical, radiographic, therapeutic, and pathologic findings. Their initial symptoms were influenza-like illness or cough with fever, and all had leukocytosis and elevation of C-reactive protein. Infectious events were ruled out by extensive bacteriologic and serologic examination. The patients' lung injury scores progressed rapidly to severe lung injury compatible with adult respiratory distress syndrome. Findings from bronchoalveolar lavage fluid showed marked neutrophilia and elevation of albumin concentrations. All patients showed various degrees of improvement following corticosteroid therapy. Histologic findings from open lung biopsy specimens showed both usual interstitial pneumonia (UIP) and organizing acute lung injury pattern. Whether these two forms of interstitial pneumonia (UIP and acute lung injury pattern) are variants of one disease or are unrelated and also the effectiveness of corticosteroid therapy on such conditions remain to be determined by further studies.