Clonal chromosome aberrations observed in patients who have relapsed after autologous bone marrow transplantation (ABMT) are usually related to the cytogenetic abnormalities observed at diagnosis. In order to assess this relationship, we evaluated 30 acute non-lymphocytic leukemia (ANLL) patients who underwent ABMT at out institution and had evaluable serial cytogenetic studies before and after ABMT. Seventeen patients (57%) showed no chromosome aberrations after ABMT in any of the studies performed, while 13 patients (43%) carried abnormalities. In eight out of 30 patients (27%0 the abnormal karyotype after ABMT was associated with hematologic relapse. The cytogenetic abnormalities were: (1) the same as at diagnosis without additional abnormalities in five patients; (2) the same as at diagnosis but with additional abnormalities in three patients. In one patient a different karyotype from that of diagnosis was detected and a myelodysplastic syndrome was clinically evaluable. Furthermore, occasional and single cell chromosome abnormalities were observed in four patients (13%), none of whom relapsed. The new and additional clonal cytogenetic abnormalities observed after ABMT were found in eight patients (27%), suggesting that this event may not be so frequent, that is presumably associated regimen. The re-appearance of the chromosome aberrations after ABMT and the relationship with the risk of relapse are discussed.