Microchimerism after allogeneic organ transplantation may be a mechanism for induction of donor-specific graft acceptance. However, the frequency of chimerism and its relevance in long-term tolerance are uncertain. We studied 15 long-surviving (more than 20 years) cadaveric-kidney transplant recipients for the systemic presence of donor alleles with allele-specific genomic amplification of DRB1 and H-Y loci. Microchimerism was observed in 1 case in peripheral blood and in 4 cases in skin. Chimerism and number of HLA alleles shared by donor and recipient were not correlated. This low frequency of microchimerism in long-term kidney allograft recipients raises doubts about a major participation of chimerism in donor-specific tolerance.