Bone marrow-derived stem cells have been shown to engraft and populate native tissues during repair and in transplanted animal tissues. Very few studies have been performed in humans to evaluate the possibility of stem cell engraftment in transplanted tissues. In human renal transplants, recipient cells have been demonstrated within vascular and interstitial structures. In a previous study of patients with hepatic transplants, hepatocytes with XY chromosome patterns have been detected in sex-mismatched female to male transplanted livers in a small number of cases. Because of the possibility of Y chromosome microchimerism of females with male offspring, we analyzed the presence of X and Y chromosomes in liver biopsies of 13 patients with sex-mismatched liver transplants (8 female to male, 5 male to female) and long transplant to biopsy intervals (1.2 to 12 years; mean, 4.5 years). We were able to detect recipient-specific sex chromosomal patterns in inflammatory cells by fluorescent in situ hybridization/immunohistochemistry combination within the liver parenchyma but not within hepatocytes. In conclusion, recipient engraftment of stem cells may be an early feature in liver transplant but may be an infrequent persistent feature in long-term grafts.