Forty patients with Ph-positive blastic phase (BP) (28 patients) or chronic phase (CP)-CML (3 patients) and relapsed adult acute lymphoblastic leukemia (ALL) (9 patients) with cytogenetical translocations [t(8;14):2 patients; t(4;8):2 patients; t(4;11):3 patients; t(9;22):2 patients], received an intensive conventional chemotherapy. During early recovery from marrow aplasia, when WBC reached 0.3-1.5 x 10--9/L, peripheral blood stem cells (BSC) were collected by 4-8 leukapheresis consecutively. BSC collected from the 2/3 patients with CP-CML resulted Ph-negative and PCR negative. In 8 out of 26 BP-CML patients, BSC resulted Ph-negative and in two cases PCR negative. Of the nine ALL patients, 6 patients lost the cytogenetic translocations, one patient died during aplasia, two patients did not have cytogenetic modifications and died in few weeks of leukemia and one patient out of six responding patients relapsed before transplant. After complete recovery, 15 patients (BP-CML:8 patients; CP-CML:2 patients; ALL:5 patients) were subsequently given high-dose therapy (VP-16 +/- Cy+TBI in single dose) followed by reinfusion of "normal" BSC. Both the patients in CP-CML and 5/5 patients with ALL maintain clinical and cytogenetic remission; all the patients transplanted in BP-CML relapsed 5-18 months post-transplant. It is concluded that intensive conventional chemotherapy employed in CML and ALL can lead to a precocious overshoot of cytogenetically normal BSC.