To investigate the effectiveness and safety of GVHD prophylaxis using FK506 alone as a continuous infusion, 104 patients who underwent reduced-intensity cord blood transplantation were retrospectively reviewed. The respective incidence of acute GVHD was 25 grade 1(24. 1%), 19 grade2(18. 3%), 15 grade3(14. 4%), and 4 grade4(3. 8%), which are comparable to that in the literature. The incidences of grade 2 and greater acute GVHD were 32 out of 69(46. 4%)for those whose wholeblood concentration of FK506 werele ss than 13 ng/mL, whereas 6 out of 35(17. 1%)for those FK5 06 were greater than 13 ng/mL. The differenceies between above and below 13 ng/mL were statistically significant(p=0. 008). There were 19 cases(18. 3%)of renal dysfunction, although none required hemodialysis. There were only 4 patients who discontinued FK506, which further confirmed the safety of FK506 alone. Together with our previous report on the upper limit of FK506(17 ng/mL)and these results, we recommend the optimal serum concentration of FK506 to range from 13 to 17 ng/ mL.