Maintenance of patency is still the primary problem with synthetic grafts replacing small blood vessels. In previous studies the importance of the type of material, the degree of tissue reaction incited, and method of construction of the grafts have been stressed.1 Little has been established regarding the porosity beyond the knowledge that grafts that are too porous will allow exsanguination, while solid tubes will almost universally become occluded by thrombosis. In a continuing search during the past 5 years for a satisfactory substitute for small blood vessels, the influence of the porosity on the fate of the grafts has become more apparent. This report is based on observations in 147 animals with crimped Teflon grafts replacing the thoracic and abdominal aortas of dogs. Materials and Methods Knitted Teflon tubes were crimped on a mandrel at elevated temperatures. The porosity was reduced to varying degrees by impregnation with a suspension