BACKGROUND Human leukocyte antigen (HLA) mismatch is a characteristic feature of post-orthotopic liver transplantation (OLT) hepatitis C. To investigate the importance of donor HLA-restricted immune cells in post-OLT hepatitis C recurrence, we analyzed the frequency of donor chimerism and the clinical course of post-OLT hepatitis C. MATERIAL AND METHODS We analyzed peripheral blood chimerism in 11 HCV-reinfected patients with post-HLA mismatched OLT. Patients were divided into 2 groups: the OLT chronic hepatitis C (CHC) group (n=8), exhibiting active hepatitis C recurrence; and the OLT-persistently normal ALT (PNALT) group (n=3), without active hepatitis. Chimerism was analyzed by flow cytometry using donor-specific anti-HLA antibodies in peripheral blood mononuclear cells from 1-100 days after OLT. Kidney (n=7) and lung (n=7) transplant recipients were also analyzed for comparison. As immune cells from the donor liver might contribute to post-OLT chimerism, the characteristics of perfusates from donor livers (n=10) were analyzed and defined. RESULTS Donor-derived cells were frequently observed in liver and lung transplant recipients. The frequency of donor-derived cells from the B cell subset was significantly higher in peripheral blood from OLT-CHC group than in that of the OLT-PNALT group. B cells, however, were not the predominant subset in the perfusates, indicating that inflow of donor-derived cells alone did not cause the chimerism. CONCLUSIONS Chimerism of B cells is frequent in liver transplant patients with early recurrence of hepatitis C. We propose that monitoring of early chimerism could facilitate early detection of chronic hepatitis C recurrence, although we need more cases to investigate.