The primary objective of this study is to define the safety and efficacy of recombinant human growth hormone (rh-GH, Genotropin) in a population of patients undergoing allogeneic stem cell transplant. The secondary objectives of this study are: to evaluate the incidence of mortality due to opportunistic infections in the first 6 months, to evaluate the incidence and severity of infectious complications, to assess laboratory parameters of post-transplant immune recovery in patients on GH therapy and to determine the probability and time of neutrophil and platelet recovery on GH therapy.