Membranous laryngotracheobronchitis (membranous croup), not previously described as a distinct entity, is characterized by diffuse inflammation of the larynx, trachea, and bronchi with adherent or semiadherent mucopurulent membranes in the subglottic trachea (conus elasticus) and in the upper trachea distal to the conus elasticus. We reviewed 28 cases of membranous croup diagnosed by endoscopy and/or radiographic examination. The importance of the recognition of membranous croup as a distinct entity is discussed. The characteristic radiologic findings consist of subglottic tracheal narrowing, irregularity of contour of the proximal tracheal mucosa, and sometimes detached or partially detached proximal tracheal membranes, which can be mistaken for tracheal foreign bodies.