We review current concepts about the clinical manifestations, diagnosis and treatment of patients with bronchiolitis obliterans (BO) with emphasis on clinical/pathological correlations and recent developments. BO is a relatively rare disease, but its incidence is probably higher than generally believed and is continuously rising, partly because of better recognition, but also because of increased exposure to industrial fumes, and its occurrence in lung transplantation. BO is characterized histologically by varying degrees of obliteration of the lumen of the respiratory bronchioles by organizing connective tissue often extending into the alveoli ('proliferative' BO with organizing pneumonia--BOOP) or by more extensive fibrosis and scarring of the more proximal, conductive bronchioles ('constrictive' BO). Diverse clinical conditions have been associated with the development of BO, notably viral and mycoplasma infection, toxic fume exposure and immune reactions in the setting of a collagen vascular disease, drug reaction or organ transplantation. The clinical course and features of BO may vary considerably according to the aetiology, histological pattern and stage of the disease. The most common presentation is that of a progressive dry cough and dyspnea, associated with diffuse patchy interstitial lung infiltrates on chest X-ray. In the more advanced cases, lung function tests show either restrictive or obstructive defects, depending on the extent of alveolar involvement, and hypoxemia without CO2 retention. The diagnosis is often possible on clinical grounds, however, in a seriously ill patient uncertainty should be resolved by tissue diagnosis, preferably by open lung biopsy. Treatment is based on symptomatic therapy. The use of corticosteroids is controversial, but common. Patients with BOOP are exceptional, in that there may be no underlying condition ('idiopathic' BOOP or cryptogenic organizing pneumonia--COP), a restrictive ventilatory defect is usual and the response to corticosteroids often remarkable.