To explore the hematopoietic reconstitution and transplantation-related complications of two units of unrelated umbilical cord blood combined transplantation for the treatment of adult hematologic malignancies, one adult patient with chronic myelogenous leukemia received two units of unrelated umbilical cord blood combined transplantation. The conditioning regimen was busulfan and cyclophosphamide (Bu-Cy). GVHD prophylaxis regimen consisted of mycophenolate mofetil (MMF), cyclosporine A (CsA) and methotrexate (MTX). The patient received total nucleated cells 4.63 x 10(7)/kg with CD34+ cells 8.34 x 10(5)/kg. Engraftment was documented by the analysis of short tandem repeat with polymerase chain reaction (STR-PCR). The results showed that the STR-PCR analysis for peripheral blood at day 31, 46 and 71 after transplantation suggested that one of two units of cord blood were completely engrafted. The ANC > 0.5 x 10(9)/L in the patient occurred at day 23, blood platelet counts > 20 x 10(9)/L at day 33 and > 50 x 10(9)/L at day 47. The Philadelphia chromosome and bcr/abl fusion gene of the patient also turned to negative after engraftment. Acute GVHD grade II occurred at day 13 and cured after treatment. It is concluded that umbilical cord blood can be used in adult hematopoietic stem cell transplantation. Two or more units umbilical cord blood combined transplantation might be the way to solve the problem of the low counts of nucleated cells when be used for adult.