Postoperative changes in the oxygen saturation of hemoglobin in the graft liver (graft SO2) were monitored by near-infrared spectroscopy in four cases complicated by hepatopulmonary syndrome. A plastic cylinder was placed in the abdominal wall, and optical measurements of the graft liver were obtained through this window. Our findings were as follows; (1) graft SO2 decreased after abdominal wall closure, and decreased further 1 day after surgery. (2) Graft SO2 was maintained despite severe hypoxemia, with partial pressure of oxygen in arterial blood as low as 50 mmHg. High hematocrit was beneficial for oxygenating the graft. (3) Graft livers could tolerate hypoxia with a graft SO2 as low as 20%. (4) It may be useful to monitor graft SO2 during the critical period after transplantation for the assessment of graft function.