A 55-year-old woman with Ph-negative acute lymphoblastic leukemia in primary induction failure received allogeneic peripheral blood stem cell transplantation from her HLA-compatible sister. Pseudohyponatremia developed due to extreme hypercholesterolemia of 4091 mg/dL accompanied by lipoprotein X and lipoprotein Y. The hypercholesterolemia was caused by cholestasis due to chronic GVHD and ischemic cholangiopathy. In addition, we found that hepatic triglyceride lipase (HTGL) activity was severely decreased, which could be another novel factor causing extreme hypercholesterolemia after allogeneic transplantation. The total cholesterol has been gradually decreasing followed by the improvement of cholestasis with bezafibrate, ursodeoxycholic acid and prednisone treatments, and by a slight increase in HTGL-protein. To our knowledge, this is the first report to describe the association of decreased HTGL with extreme hypercholesterolemia after allogeneic transplantation.