The porosity of a vascular graft material has been suggested as a major factor affecting the rate and degree of neovascularization of newly implanted grafts, with higher porosites generally associated with better performance. The objective of this study was to determine the water porosity of a new vascular graft material, small‐intestinal submucosa (SIS), and to compare the values to those reported for other common vascular graft materials. In addition, the porosity of SIS was investigated with respect to applied pressure and applied uniaxial tension. Both rectangular, flat specimens and tubular specimens of SIS were subjected to static water pressure, and water was collected as it passed through the SIS material. SIS has a typical porosity of 0.52 mL/min · cm−2 at an applied pressure of 120 mm Hg. Although porosity appeared to be unaffected by uniaxial tension, it increased in proportion to applied pressure at a rate of 4.8 × 10−3 mL/min · cm−2/mm Hg. Theselow porosity values and the past success of SIS as a vascular graft material suggest that high‐porosity materials are not required for implant succes. © 1993 John Wiley & Sons, Inc.