Previous animals studies have demonstrated a fall in liver blood flow associated with acute rejection after liver transplantation. To study the relationship between rejection and liver blood flow in humans after liver transplantation indocyanine green (ICG) clearance was measured serially in 7 patients with clinical and histological features of acute rejection. There was a consistent pattern of satisfactory initial ICG clearance that fell in association with acute rejection and rose with successful treatment of the episode of rejection. In one patient there was no improvement in ICG clearance after treatment with additional immunosuppression, and she subsequently required retransplantation for chronic rejection. The volume of distribution of ICG was also estimated and fell considerably during the first weeks after transplantation. These results show that rejection is associated with a reduction in ICG clearance that may be due to a fall in liver blood flow, and that graft ischemia and rejection may therefore be interrelated, and important in one another's etiology.