PURPOSE: The umbilical cord blood has been considered as an alternative source of hematopoietic stem cells for transplantation. The CD34+ is known as a common stem cell antigen and the CD34+ CD38- immunophenotype reportedly defines a primitive subpopulation of progenitor cells. In this study the frequency of CD34+ and CD34+ CD38- hematopoietic stem/progenitor cells in placental/cord blood (UCB), and adult peripheral blood (PB) were determined. METHODS: Between July and September 1998, 27 collections of UCB were performed at the obstetric units of Hanyang University Hospital. Fifteen adult PB samples were also obtained for control. RESULTS: The frequency of total CD34+ cells in UCB was higher (1.13+/-0.58% vs 0.46+/-0.30%, P=0.0002) than PB and the frequency of CD34+ CD38- cells in UCB was also greater (0.02+/-0.02 vs 0.01+/-0.01, P=0.035) than PB. The majority of UCB- and PB-derived CD34+ cells expressed CD38- antigen (98.22+/-2.11% in UCB and 97.85+/-2.56% in PB). The frequency of CD38- expression by UCB derived CD34+ cells was slightly higher (not statistically significant) than that by PB derived CD34+ cells. The frequency of CD34+ cells was increased linearly with birth weight (r=0.413). CONCLUSION: These results suggest that UCB could be a useful source of highly primitive hematopoietic stem cells for marrow reconstitution after bone marrow ablation.