Objective To study the syndrome of inappropriate ADH secretion(SIADH)after all ogeneic hematopoietic stem cell transplantation(allo-HSCT)and the possible etiology.Methods SIADH was occurred in one patient with acute lymphoblastic leukemia who received secondary haploidentical stem cell transplantation after graft failure following umbilical cord blood transplantation.Results An 11-year-old boy was diagnosed as high-risk acute lymphoblastic leukemia,and the transplantation of unrelated double umbilical cord blood was performed for the first time,however,there was no engraftment at 28 days after the first transplantation.Secondary haploidentical stem cell transplantation was administered at day 33 after the first transplantion.The bone marrow and the peripheral blood stem cells from the patient's father transplantation was adopted.Pre-engraftment syndrome was developed in the patient at day 7 after secondary haploidentical HSCT followed by severe hyponatraemia(lowest serum sodium 115.8 mmol/L),natriuresis,hypo-osmolality of plasma,and twitch at day 26 after allo-HSCT.Then SIADH was diagnosed.The clinical condition was improved after restriction of water and administration of hypertonic saline,and eventually SIADH was controlled completely.By now,the patient had lived free of disease more than one year,with a normal diet and serum sodium in normal ranges.Conclusions SIADH after allo-HSCT is a rare fatal acute complication of central nervous system,whic h are probably associated with numerous transplant-related causes.Early accurate diagnosis and treatment promptly are great importance.