Extensive tracheal defects after prolonged assisted ventilation, trauma or large resections in tumor surgery are a challenge in plastic and reconstructive surgery. Defects which cannot be satisfactorily repaired reguine near of an alloplastic tracheal replacement. Previous experimental and clinical experiences in the development of an alloplastic tracheal prosthesis have demonstrated that the main cause for failure is the lack of an epithelial lining of luminal surfaces and inadequate biophysical properties of the prosthesis. With the use of a cell-seeding technique tested in vitro on biomaterials epithelialization of tracheal prostheses can be tested in vivo. In animal experiments isolated respiratory cells were seeded into implanted tubular prostheses of porous polyurethane or expanded polytetrafluorethylene. Light and scanning microscopic investigations then showed the tendency of epithelialization to occur on the luminal surfaces. Vigorous squamous epithelium cell layers that were single and (predominantly) multiple layers were found. Differentiated cilated or mucous cells were not detected in any case. The present results have shown that epithelialization of incorporated porous implants is possible. The realization of usable tracheal replacement for clinical practice must still be tested in further experiments.