Bronchial casts are characterized by the formation of obstructive airway plugs that may be large enough to fill the branching pattern of an entire lung. The condition is rare but can occur at any age. Casts may be secondary to underlying diseases such as asthma and cystic fibrosis, but there are often no predisposing factors. There is no accepted classification system for bronchial casts; but only a confusion of descriptive terms such as mucoid impaction, fibrinous bronchitis, and pseudomembranous bronchitis. Based on a review of nine well-documented cases and the available literature, we have separated bronchial casts into two well-defined groups: Type 1 (inflammatory), consisting of casts composed mainly of fibrin with a dense eosinophilic inflammatory infiltrate; and Type 2 (acellular), consisting of casts composed mainly of mucin with little or no cellular infiltrate and occurring only in children with congenital cyanotic heart disease. Acute mortality was high in both groups. Survivors of Type 1 casts seem to be well controlled with inhaled steroids. Optimal therapy for patients with Type 2 casts is not clear; the prognosis probably depends on underlying cardiac status. We hope that this simple classification will provide a framework for further study of this obscure condition.