Bird fancier's lung (BFL) is one of the most common types of hypersensitivity pneumonitis. We report a rare case of acute-on-chronic bird fancier's lung that developed in a pigeon breeder and presented subpleural curvilinear shadow and ground glass opacity on high-resolution computed tomography (HRCT) of the chest. The results of surgical lung biopsy showed mainly intraalveolar organization and alveolitis in addition to the pattern of usual interstitial pneumonia with centrilobular fibrosis. Examination of bronchoalveolar lavage (BAL) fluid revealed an increase in lymphocytes. The results of immunoglobulin (Ig) G and IgA antibodies against pigeon dropping extracts were positive in sera and BAL fluid. Consequently, the patient was diagnosed as having BFL. Avoidance of pigeons and corticosteroid therapy led to rapid improvement.