We describe a case of extreme hypercholesterolemia, mediated by lipoprotein X, in a 12-year-old Caucasian female who underwent an unrelated allogenic bone marrow transplant for relapsed acute myelocytic leukemia (AML). Her post-transplant course was complicated by severe chronic graft-versus-host disease (GVHD) of the liver. Previously normal serum cholesterol and triglycerides rose to 1,122 mg/dl (29.0 mmol/L) and 1,100 mg/dl (12.4 mmol/L), respectively. Serum cholesterol appeared to be dominantly carried by lipoprotein X. Intra-hepatic cholestasis leading to reflux of bile lipoproteins into the blood stream and subsequent formation of lipoprotein X appears to be the mechanism underlying this phenomenon. Pediatr Blood Cancer 2008;50:1280–1281. © 2008 Wiley-Liss, Inc.