Despite potent intensive conditioning regimens, hematopoietic stem cell transplantation (HSCT) may fail because of either relapse of the malignancy or the rejection of the graft. We report on 27 pediatric patients who received a second HSCT from an allogeneic donor for relapsed malignancy or graft failure. One-year, 5-year, and 10-year probabilities of survival for all patients were 53%, 36%, and 24%, respectively. Twenty patients received second HSCTs for relapsed malignancy, of whom 6 were alive and disease free at the time of this report. Seven patients received a second HSCT for graft failure, of whom 3 were alive and well as of this report. Twenty-five patients were tested for immune reconstitution following their second HSCT. Sixteen patients developed antigen-specific T-lymphocyte responses; the median time to development of antigen-specific responses was 13 months. There was no significant neurocognitive decline in patients tested 1 to 3 years following their second HSCT. Endocrine evaluations revealed deficiencies in growth hormone (7 patients), gonadal function (3 patients), and thyroid function (2 patients). Three patients developed significant abnormalities of tooth development, including absence of secondary teeth. These results show that a second HSCT offers curative therapy for selected pediatric patients whose first HSCT failed. Although toxicity is considerable following a second transplantation, the major causes of mortality continue to be relapse and infection.