This study was to investigate the reconstitution of NK cells and their receptors after unrelated cord blood stem cell transplantation (UCBT) and its clinical importance. 11 cases of acute leukemia underwent UCBT were enrolled in this study. The reconstitution of NK cells and their surface receptors as well as the the recovery of T and B cells within 90 days after clinical engraftment following UCBT were measured and analysed by flow cytometry. The results indicated that the recovery of NK cells appears to be relatively early. CD3(-)56(+) NK cell count was (35.12 +/- 18.66)% of peripheral blood (PB) lymphocytes on the day of clinical engraftment and higher than that in normal. The peak of the NK cells reached to (37.8 +/- 17.52)% of lymphocyte at 30 days after clinical engraftment. NK count was (30.4 +/- 19.14)% at 60 days after clinical engraftment when the absolute NK cell count reached to the peak (up to 544 cells/microl) in PB. The activated receptor NKG2D was reconstituted fast and high expressed [(79.58 +/- 8.71)%] at the time of clinical engraftment with a tendency of gradual elevation, which reached to peak value (82.55 +/- 9.10)% at day 60. Another activated receptor NKp46 also reconstituted fast, and maintained at a high level even at 90 days after clinical engraftment. The expression of NKG2A was lower than that of the activated receptor of NK cells, which tendency lasted for at least 90 days after clinical engraftment. The reconstitution of T cells in PB after UCBT was relatively slow with lower expression rate. It is concluded that the reconstitution of NK cells in patients with acute leukemia is earlier following UCBT. The earlier recovery of activated receptor of NK cells, especially NKG2D, suggests that the activation of NK cells may play a role in graft versus leukemia (GVL) effect in the early period after UCBT.