Pulmonary lesions are common in a variety of diseases which have hypersensitivity as their basic component. We have categorized these on the basis of the site of primary reaction.In bronchial asthma, the airway is involved.The supporting structures of the lung are the site of primary response in bagasse workers' disease, progressive systemic sclerosis, rheumatoid lung, eosinophilic granuloma and desquamative interstitial pneumonia.The vascular elements are the site of basic reaction in hypersensitivity to such drugs as nitrofurantoin and penicillin, systemic lupus erythematosus and polyarteritis nodosa.Wegener's granulomatosis is best understood as a hypersensitivity response of complex nature. The pulmonary lesions are variable and nonspecific, but the roentgenologist is in a favorable position to suggest a diagnosis of hypersensitivity response if he keeps in mind the potential variability of pulmonary lesions and evaluates each case in the light of knowledge about the clinical course of the disease.