We report a case of acute exacerbation of idiopathic pulmonary fibrosis (AE-IPF) after pandemic influenza (H1N1) vaccination. A 57-year-old man, who had been diagnosed with IPF in September 2008, was admitted to our hospital in December 2009 because of aggravation of dyspnea and fever two days after H1N1 vaccination. Chest computed tomography showed diffuse bilateral ground-glass opacities superimposed on preceding reticular opacities. We diagnosed AE-IPF. Corticosteroid and cyclophosphamide were effective. Although the efficacy of influenza vaccination in patients with chronic lung diseases is well established, physicians should keep in mind that influenza vaccination has the potential to cause AE-IPF.