Fluorescent in situ hybridization (FISH) is a reliable, rapid, sensitive, and quantitative method for detection of residual host cells following sex-mismatched bone marrow transplantation. Recently, donor-derived long-term multilineage hematopoiesis was detected in a sex-mismatched liver transplant recipient. We therefore assessed chimeric status in 12 patients (F 9, M 3), mean age 42.5 years (range 16-57), for a median period of 18 months (range 7-32) following sex-mismatched liver transplantation. Peripheral blood he-matolymphoid cells were hybridized with Y- or X-chro-mosome fluorescently labeled specific probes, and the donor-typed hematopoietic cells were enumerated. In two F recipients 4-5% male hematolymphoid cells were detected in the peripheral blood at 15 and 22 months after sex-mismatched liver transplantation, respectively. These two patients with systemic chimerism suffered from primary biliary cirrhosis and fulminant Wilson's disease before transplantation. One of them had evidence of graft rejection only once during the posttransplant course and the other had no episode of graft rejection. Two other female patients who were found to have 2% male hematolymphoid cells, which is considered to be in the false-positive range, also had no signs of graft rejection during the post-transplant follow-up period. Among the remaining eight patients, in whom systemic chimerism was un-detectable, there was at least one episode of acute cellular rejection during the posttransplant period. In summary, the FISH technique enables us to detect systemic chimerism following sex-mismatched liver allografts. Inasmuch as balanced systemic chimerism after organ transplantation is of major importance for self tolerance, our findings may enable us to treat patients after liver transplantation without the need for immunosuppression.