Small‐intestinal submucosa (SIS) has been shown to be a promising biomaterial for vascular graft applications. This study examines the directionality property of SIS porosity using 35 SIS specimens from 13 pigs. In addition, the effects of the weight of the donor pig, preconditioning of 13 additional SIS specimens, and the duration of the test of five additional SIS specimens on such porosity are reported. The porosity from serosal to mucosal direction was found to be four times greater than the porosity in the opposite direction. The weight of the donor pig was not found to be an important factor in SIS porosity. Preconditioning served to increase the average serosal porosity index at 120 mm Hg static water pressure from 2.99 to 8.33 mL/(min cm2). The porosity in the mucosal direction was not affected by preconditioning. Porosity in both directions decreased with increasing test duration. The directionality property of SIS porosity may be an important factor in its success as a vascular graft. The term ‘porosity’ is used throughout this article, but current standards also refer to the term ‘permeability’ to describe the passage of liquid through a vascular graft. © 1993 John Wiley & Sons, Inc.