Serial cytogenetic studies were performed on 60 leukemic recipients of HLA-matched bone marrow transplants (BMT) who were prepared by high doses of alkylating agents and fractionated total body irradiation (TBI). Forty-three patients were recipients of untreated BMT and 17 were recipients of T-depleted BMT. Donor or host mitoses were identified by examination of sex chromosomes in 54 patients or by evaluation of the polymorphism of other chromosomes after specific banding in six patients. Mixed lymphoid chimerism (MLC) was identified in 29 patients and full donor lymphoid chimerism (FDLC) in 29 patients. Complete donor hematopoiesis was recovered in most patients after 12 months, but two T-depleted patients had persistent host cells at 46 and 52 months after the graft. Acute graft-versus-host disease was significantly less frequent in patients with MLC, especially when more than 10% of residual lymphoid cells were detected. The probability of relapse and survival was identical in patients with MLC and FDLC, except in patients with chronic myeloid leukemia where MLC was significantly associated with an increased risk of relapse.