Hypersensitivity pneumonitis is an immunologic mediated response in the lung to inhaled organic antigen. Farmer's lung and pigeon breeder's lung are the two most widely know disorders. The chest radiograph usually is normal in hypersensitivity pneumonitis. When abnormal, the distribution of disease favors the lung bases with acute disease and the upper lung zones with chronic disease. Computed tomography (CT) may be more sensitive in detecting parenchymal abnormalities, although the characteristic findings of hypersensitivity pneumonitis at CT or high-resolution CT are nonspecific.