Reactive polyurethane was investigated for use as coating material and compared with other materials which are presently employed in clinical use. Reactive polyurethane is a viscous li-quid which reacts with amine and water resulting in an elastomeric substance within a few minutes. After application to an aneurysm, which was made by venous pouch of the carotid artery in cats, the effect of this material as a coating material was examined macroscopically and microscopically. Other coating materials including Biobond, fibrin glue and Bemsheet were also examined. The fibrin glue was not found around the aneurysm three months after application and the amout of Biobond had substantially decreased six months after application. Bemsheet was found to coat the aneurysm well but was not sufficiently firm. The amount of reactive polyurethane had not decreased six months after application, and microscopic examination showed that the fibrous tissue had infiltrated into the many small holes which were made when it reacted with water on application, resulting in good adhesiveness between the wall of the aneurysm and the polyurethane. These results indicate the feasibility of reactive polyurethane as a coating material.