Interferon (IFN)-gamma was recently proposed as a treatment for idiopathic pulmonary fibrosis. We report on four patients who developed acute respiratory failure with new alveolar opacities after 2 (two patients), 6, and 35 injections of IFN-gamma-1b. All four patients had advanced idiopathic pulmonary fibrosis (total lung capacity less than 45% predicted or carbon monoxide diffusion capacity less than 30% predicted), and two patients had familial pulmonary fibrosis. No other cause of deterioration was found. Refractory hypoxemia led to death in three cases and to lung transplantation in one case. Pathologic studies in two patients showed diffuse alveolar damage lesions with preexisting usual interstitial pneumonia. These cases suggest that IFN-gamma therapy can induce an acute respiratory failure in patients with end-stage idiopathic pulmonary fibrosis.