The nomenclature of the bronchiolitides is complicated by the interchangeable use of pathological and clinical descriptions and a diversity of classification systems. The four primary histological patterns are: organizing pneumonia (also termed proliferative bronchiolitis and bronchiolitis obliterans organizing pneumonia); bronchiolitis obliterans (also termed obliterative bronchiolitis and constrictive bronchiolitis); follicular bronchiolitis; and diffuse panbronchiolitis. The most characteristic abnormality is a filling of alveoli with granulation tissue and buds of loose collagen and connective tissue matrix cells with a uniform appearance. Presentation is typically subacute with non-productive or minimally productive cough, insidious dyspnoea, and systemic symptoms including malaise, fever, or chills, weight loss, and myalgia. Clinical signs are non-specific. Corticosteroid therapy is usually effective, with other immunosuppressive agents given to fulminant cases or those that do not respond. Prognosis is usually good, with overall mortality less than 5%.