Pulmonary disease was studied in four patients with ataxia-telangiectasia. Immunodeficiency was characterized by lymphopaenia, hypo-gammaglobulinaemia and decreased T-cell response to phytohaemagglutinin stimulation in mixed lymphocyte cultures. All four patients died from respiratory failure. Autopsy revealed that all four patients suffered from bronchiolitis obliterans in all lobes. Immunohistochemical examination demonstrated expression of MHC class II antigens on bronchiolar epithelium. Pulmonary infections in ataxia-telangiectasia patients included a case of mycoplasma pneumonia, one of cytomegalovirus pneumonia and one of Pseudomonas aeruginosa infection. The aetiology and immunological background of bronchiolitis obliterans are discussed. Bronchiolitis obliterans is a characteristic finding in ataxia-telangiectasia and may be due to the underlying immune deficit.