The features of the engraftment in 26 patients allografted using reduced-intensity conditioning regimen (8 with chronic myelogenous leukemia, 6 with acute myelogenous leukemia, 9 with acute lymphoblastic leukemia, 1 with hybrid acute leukemia, 1 with myelodysplasia and 1 with thalassemia major) were analyzed. Patients received a median of 10 x 10(8)/Kg mononuclear cells (range 1.6 to 22.9), and a median of 4.2 x 10(6)/Kg CD34 cells (range 0.3 to 14). There was a linear correlation between the number of infused mononuclear cells (MNC) and that of CD34 cells (r = 0.78, p = 0.002). Three patients (11%) failed to engraft; in those who engrafted, the median time to achieve > 500 granulocytes was 11 days (range 10 to 22), and the median time to achieve > 10,000 platelets was 12 days (range 10 to 41). The three patients who failed to engraft received less than 5 x 10(8)/Kg MNC (1.6, 4.6 and 4.9) and less than 0.5 x 10(6)/Kg CD34; however, five of eight patients who received less than 5 x 10(8)/Kg MNC still engrafted successfully. On the other hand, all the patients who received less than 0.5 x 10(6)/Kg CD34 cells failed to engraft. Within the group of patients who engrafted, it was found that those who received more than 7 x 10(6)/Kg CD34+ cells tended to earlier recover > 20 x 10(9)/L platelets (p = 0.02), and > 0.5 x 10(9)/L neutrophils (p = 0.06) before day 15, than those who received less than 7 x 10(6)/Kg CD34+ cells. No such association could be established between the number of MNC and the time for recovery. In these patients allografted using reduced-intensity conditioning regimens, the target doses of hematopoietic cell used were similar to those described for conventional allografts: The number of CD34 infused cells was significantly related to the possibility of failure to engraft and to the recovery rate of the hemopoiesis.