BACKGROUND Small-diameter prosthetic H-graft portacaval shunts have been shown to promote preservation of total hepatic blood flow relative to large-diameter H-graft shunts. Nonetheless, specific changes in portal hemodynamics occurring with small-diameter H-graft shunting are unknown. This study was undertaken to evaluate changes in portal flow that occur with these shunts. METHODS Portal vein and inferior vena cava (IVC) blood flow were determined intraoperatively in 36 consecutive adults before and after prosthetic H-graft portacaval shunting using color-flow Doppler ultrasound. Postshunt measurements were made immediately cephalad and caudad to the shunt. Comparisons were undertaken using a paired Student's t-test with 95% confidence. RESULTS Portal pressures decreased in all (P < 0.001), but never to normal. Postshunt portal flow cephalad and caudad to the shunt were not different from preshunt flow (P = 0.09, P = 0.28, respectively), although they were different from each other (P = 0.004). Postshunt IVC flow cephalad to the shunt was greater than caudad IVC flow (P = 0.004) and greater than preshunt IVC flow (P < 0.001), reflecting high flow through the shunt into the IVC. CONCLUSIONS Small-diameter prosthetic H-graft portacaval shunts divert a significant amount of blood from the portal vein and significantly decrease portal pressures. The decreases in portal pressures with shunting are significant whereas changes in portal blood flow into the liver are not. These findings help explain the low incidence of variceal rebleeding and hepatic dysfunction after these shunts.