In this study, a high CD34% in autologous peripheral blood stem cell (PBSC) products from 71 AML patients was associated directly with a high relapse rate (P = 0.006) and inversely with disease-free survival (P = 0.003), irrespective whether patients were transplanted or not. The relapse rate at 12 months was 67% in a group with >0.8% CD34+ cells and 34% in a group with < or = 0.8% CD34+ cells. Although the percentage of malignant CD34+ cells in the CD34+ compartment in the relapses of the first group was not high (median 8%), the total number of malignant cells as a percentage of WBC was about 13 times higher than for the patients remaining >12 months in remission. When all patients evaluable were taken together, this frequency of malignant cells correlated strongly with disease-free survival (P < 0.001). Both this massive mobilization of normal CD34+ cells and high frequency of malignant cells in the subgroup of patients with CD34 >0.8% and relapse within 12 months indicate an insufficient in vivo purging, as well as low chemotherapeutic bone marrow toxicity. This was confirmed by an inverse correlation between hypoplasia period after the induction therapy and CD34% in PBSC products (P < 0.002). It is concluded that a subgroup of patients has been identified that might benefit from a more intensive chemotherapeutic treatment.