During conventional follow-up of patients with acute myeloid leukemia (AML), the emergence of cytopenias is considered to be a sign of impending relapse, and it represents an example of how leukemic hematopoiesis affects normal hemopoietic differentiation. In the present study, we have explored the possible value of the analysis of the distribution of CD34 ؉ myeloid and CD34 ؉ lymphoid progenitor cells in follow-up complete remission bone marrow samples from de novo AML patients as a prognostic parameter for predicting relapse. A total of 213 bone marrow samples from 36 AML patients in morphological complete remission, obtained at the end of induction, consolidation, and intensification therapy and every six months thereafter were analyzed. The normal CD34 ؉ myeloid/CD34 ؉ lymphoid ratio ranged between 2.4 and 8.9. In contrast, in most AML cases an abnormally high ratio (H10) was observed at the end of induction and consolidation therapy: 96% and 75% of cases, respectively. On the other hand, at the end of intensification, 70% of the patients displayed a normal CD34 ؉ ratio. Patients with a myeloid/lymphoid CD34 ؉ ratio higher than 10 at the end of intensification showed a significantly lower overall survival (median survival of 19 months versus median not reached, P ؍ 0.05), as well as a lower disease-free survival (median of 7 months versus 30 months, P ؍ 0.0001). Regarding sequential studies, 67% of the relapses were preceded by the re-appearance of an abnormal CD34 ratio, whereas relapse was not predicted in four patient with leukemia classified as M 3 undergoing maintenance therapy. From the remaining 18 patients who are still in continuous complete remission, all except 3 cases (17%) displayed a normal CD34 myeloid/lymphoid ratio. In summary, the present study shows that the persistence at the end of chemotherapy of an abnormally high (H10) ratio between CD34 ؉ myeloid and CD34 ؉ lymphoid progenitors in the bone marrow of AML patients is associated with high risk of relapse and a shorter overall survival. Cytometry (Comm. Clin. Cytometry) 38: 70-75,