We performed polymerase chain reaction-variable number of tandem repeats analysis of flow-sorted peripheral blood T-, B-, natural killer- , and myeloid cell populations (van Leeuwen et al, Br J Haematol 79:218, 1991) in 32 children following allogeneic bone marrow transplantation (BMT) for leukemia to evaluate the relationship between mixed lymphoid chimerism and leukemia relapse. Five patients showed a stable mixed chimerism pattern characterized by the presence of both recipient as well as donor type cells in all cell populations up to 1 year posttransplantation. Five others showed transient mixed chimerism in the T-lymphoid cell lineage. In one patient, host T cells persisted until leukemia relapse. The remaining 21 patients showed a complete chimerism throughout the period of investigation. Twenty-five of these patients were classified according to the presence (n = 10) or absence (n = 15) of recipient type T cells. Statistical analysis did not show significant differences in the distribution of a number of clinical variables between the two groups, nor in the actuarial survival (P = .11) and leukemia-free interval (P = .97). Therefore, these results suggest that persistence of recipient type T lymphoid cells after allogeneic BMT for hematologic malignancies is not correlated with leukemia relapse. In addition, we observed that persistence of host cells within the original leukemia cell lineage and at the correct maturational stage was predictive for leukemia relapse in one case.