Many of the diseases associated with bronchiolitis may produce varying degrees of both histologic manifestations--i.e., proliferative and constrictive bronchiolitis. The factors responsible for producing one pattern or the other are not known. It therefore is important that one consider both the etiologic and histologic classification when attempting to identify a case of bronchiolitis. In some instances, it appears that proliferative bronchiolitis, the more common variety, is an "early" lesion that may completely or partially resolve. Constrictive bronchiolitis is a rare finding and may be the final outcome of persistent and severe proliferative bronchiolitis. Most often, constrictive bronchiolitis appears to be a separate process dependent on the type, extent, and severity of the initial insult.