BACKGROUND The clinical course of patients with idiopathic pulmonary fibrosis (IPF) is generally marked by a decline in pulmonary function over time. Increasingly, patients have been recognized as having an acute, and often fatal, clinical deterioration, termed an acute exacerbation of IPF (AE-IPF). METHODS Review of the current literature pertaining to AE-IPF. RESULTS Acute exacerbations are defined by an acute onset of dyspnea (<1 month) with worsening hypoxia and progressive infiltrates seen in the absence of heart failure or infection. New ground-glass infiltrates are seen on chest CT scans with diffuse alveolar damage superimposed on a background of usual interstitial pneumonia that is evident on histopathology. The incidence is unknown and is impeded by difficulties in eliminating infection as a cause, as well as by reporting biases contained in reported series introduced by including only biopsied patients or only deaths, or by excluding patients with advanced disease. Prognosis is poor but may be influenced by diagnostic inaccuracy. Treatment with antiinflammatory therapies, such as corticosteroids, or with anticoagulation are unproven and have not as yet been fully studied. CONCLUSIONS AE of IPF is a complication that demands additional careful study to clarify its relationship to the clinical course of patients with IPF.