In children receiving a hematopoietic stem cell transplant (HSCT), blood levels of TNFalpha (an inflammatory cytokine) at the onset of the acute GVHD (graft-versus-host disease) could be correlated with the severity of the disease. The hypothesis is that the highest infliximab (a biologic drug against TNFalpha) could be associated with a significant reduction in TNFa levels and, subsequently, with a faster remission of the symptoms and prevention of disease progression. Moreover, a rapid drop of infliximab serum concentration, documented by therapeutic drug monitoring (TDM), could be related to the active phase of GVHD and higher production of TNFalpha. Therefore, the study is aimed at investigating whether the drop in infliximab plasma concentrations could be associated with clinical response and production of TNFalpha.||HSCT children receiving infliximab to control GVHD are enrolled. Blood samples will be collected during treatment and they serve to measure drug and TNFalpha concentrations. Drug levels are analyzed by a population pharmacokinetic modeling and results are compared with plasma concentrations of TNFalfa and clinical response.