Both in vivo and mathematical models of venous hypertension were used to evaluate the hemodynamic effects of 4, 6, 8, and 10 mm diameter cross-femoral venous bypass grafts (CFBs). Eighteen grafts (length 138 +/- 3.4 mm) were tested in paired sequential fashion (four grafts, 4 and 8 mm; five grafts, 6 and 10 mm) in nine greyhounds (femoral vein diameter, 7.7 +/- 0.09 mm). Bilateral hindlimb venous pressures and flows were measured before and after unilateral iliofemoral venous ligation, 30 minutes after CFB insertion, and for 5 minutes after venous flow augmentation induced by stimulated muscle contraction. CFBs of all sizes were equally effective at relieving the occlusive venous hypertension at rest. Muscle contraction elevated venous pressure in all ligated hindlimbs (p less than 0.0001); however, the pressure returned to baseline by 3 minutes in dogs with 6, 8, and 10 mm grafts but remained elevated (p less than 0.05) with the 4 mm grafts even after 5 minutes. Peak graft flow (first 90 seconds after contraction) was significantly greater through the 8 mm grafts than through the 4 mm grafts (p less than 0.01), although no difference was noted in flow rates between 6 and 10 mm grafts. The pressure gradient across the graft as predicted by the mathematic model for 6 to 10 mm conduits was less than 5 mm Hg for flows up to 1000 ml/min, although the pressure gradient of the 4 mm graft exceeded 5 mm Hg at 200 ml/min and approached 30 mm Hg at 1000 ml/min. Therefore, data from both canine and mathematical models agreed that, at rates approximating human resting flow (1000 ml/min), no adverse short-term hemodynamic consequences result from CFB conduits of 6 to 10 mm diameter.