The authors report on a sequential cytogenetic study carried out on 31 patients with acute leukemia (20 with acute lymphoblastic leukemia and 11 with acute non-lymphocytic leukemia) who underwent bone marrow transplantation (BMT). Engraftment was documented in all patients with sex-mismatched donors and with donor constitutional aberrations. During the follow-up, ranging from 6 to 110 months, clinical and hematologic relapse was observed in 11 patients (35.5%). Five of these cases showed a normal karyotype, 3 were of undefined relapse origin, 2 were aneuploid karyotypes, and one was donor (male) metaphases. Cytogenetic and immunologic data in the latter patient were suggestive of relapse in donor cells.