 Study samples Patient randomization will be performed by the coordinating study centre in Pavia; Dr. M. Zecca will be the statistician responsible for the study. Patient enrolment was calculated by a sample size evaluation method based on the log rank test to estimate the difference in the cumulative incidence of grade II-IV acute GVHD. The minimum number of patients to be randomized was 80 per arm based on a significance level of 0.05, a study power of 0.80, and hypothesizing, for children in the lower dosage arm a grade II-IV GVHD incidence of 50 and for children in the higher dosage arm a grade II-IV GVHD incidence of 25. Interim analysis will be performed after recruitment of the first 10, 20 and 50 patients. 