Hematopoietic stem cell transplantation is used as a common medical procedure for the treatment of numerous hematological, immunological illnesses and solid tumors in children. Hematopoietic stem cells are immature cells with the ability to proliferate and differentiate into all types of blood cells. The source of hematopoietic stem cells is the bone marrow, peripheral blood and the umbilical cord. Allogeneic hematopoietic stem cell transplantation (allo – HSCT) is a procedure in which the patient receives hematopoietic stem cells from a donor. Before the allo-HSCT patients must go through a conditioning treatment (chemotherapy and/or radiotherapy) to remove less valuable host hematopoietic cells (tumor or genetically changed cells), to create space for healthy cells from the transplant and to prevent the host's immune system from rejecting the transplant. The donor must be compatible in the histocompatibility antigens (HLA) system. The patient may have a sibling with a matching HLA system (related donor) or an unrelated donor with an identical HLA system. Unrelated donors can be found in the registry of voluntary hematopoietic stem cell donors. Treatment with allo-HSCT is divided into three phases: pretransplantation phase, transplantation and posttransplantation phase. Possible complications from allo-HSCT treatment are primarily infections, GvHD and early and late toxic complications due to the conditioning treatment in the pretransplantation and immunosuppressive treatment in the posttransplantation period. The aim of this study was to analyze the results of treatment with allogeneic HSCT at the Division of Pediatric Hematology and Oncology, Department of Pediatrics, School of Medicine and University Hospital Center Zagreb. The data were retrospectively analyzed on the sample of 49 children which were treated with allogeneic HSCT in the period between January 2009 and December 2014 at the Division of the Pediatric Hematology and Oncology, Department of Pediatrics, School of Medicine and University Hospital Center Zagreb.