Improved hepatic microcirculation is one of the postulated mechanisms by which prostaglandin E1 (PGE(1)) could improve liver function in the setting of fulminant hepatic failure or poor allograft function immediately post-transplant. The purpose of this study was to determine whether PGE(1) improves hepatic allograft arterial and portal vein inflow. Fifty patients receiving a primary liver transplant were entered into a prospective randomized double-blind study. Measurements of hepatic arterial and portal venous flow were taken immediately after revascularization and after steady state drug infusion. PGE(1) (or placebo) was infused peripherally at an increasing dose and titrated for blood pressure to a target dose of 1 microgram/kg/hr (80 microgram/hr maximum). Changes in hepatic arterial and portal vein flow, and time to reach maximum dose were compared using the student's t test. Of the 50 patients entered, 15 were excluded from analysis (10 because of inability to obtain flow measurements, 4 because they were pediatric recipients <2 yrs of age, and 1 patient who was placed on open PGE(1)). Thirty-five patients were available for final analysis (control, N = 14; PGE(1),N = 21). There were no statistical differences in the change in hepatic artery of portal vein flow between PGE(1)-treated and -untreated grafts. The time to reach maximum dose of PGE(1) was longer in the PGE(1) treatment group. Maximum dose was reached in 85.7 per cent of patients in both groups. There was an effect on blood pressure in the study group as demonstrated by a longer time to reach maximum dose. There was no effect on hepatic vascular in flow.