The paper compares ultrastructural regeneration of the tracheal epithelium on patch grafts coated or not coated with collagen. A longitudinal patch window 2 x 1 cm (four cartilaginous rings in cephalocaudal extent) was made on the ventral wall of the cervical trachea in dogs, and replaced by a polypropylene Malex mesh graft. Grafts coated with collagen allowed normal connective tissue ingrowth and subsequent epithelial spreading. From the cut edge, flat, stratified, poorly differentiated cells migrated and spread on the newly formed stroma. Six weeks after the operation, the prosthesis was thoroughly covered by pseudostratified ciliated columnar epithelium with a newly formed basal lamina. On uncoated grafts, in contrast, some regenerated ciliated cells formed compound cilia or irregular microvilli and dilated endoplasmic reticulum. At 6 weeks, no normal basal cells differentiated, although some bizarre flat cells with extremely extended cytoplasm were located in the basal area of the epithelium. The basal lamina was thick and discontinuous. The underlying stroma included abnormally elongated fibroblasts with condensed cytoplasm and curved, randomly dispersed collagen fibrils. These ultrastructural results indicate that (1) regenerated epithelial cells were derived from poorly differentiated cells; (2) a plastic implant may lead to abnormal regeneration in the connective tissues and epithelium; and (3) collagen coating of the graft may allow fibroblasts to produce normal connective tissue substances.