We evaluated the outcome of second allogeneic bone marrow transplantations (BMTs) in 59 patients aged 1-57 years who relapsed after initial autologous transplantation. Patients received a second transplantation for recurrent acute myeloid leukemia (AML) (n = 24), acute lymphoblastic leukemia (ALL) (n = 13), lymphoma (n = 18), multiple myeloma (n = 3), or chronic myelogenous leukemia (n = 1) from an HLA-matched related (n = 14), mismatched related (n = 25), or matched unrelated (n = 20) donor. The probabilities of nonrelapse mortality, relapse, and disease-free survival (DFS) 2 years after the second BMT were 51%, 26%, and 23%, respectively. The 2-year DFS estimates for AML, ALL, and lymphoma were 46%, 23%, and 0%. Univariate analysis demonstrated that superior DFS was associated with age < or =17 years at the time of the second transplantation, remission before the second transplantation, total-body irradiation-based preparative regimen for the second transplantation, and the diagnosis of AML. These data demonstrate that an allogeneic transplantation after a failed autologous transplantation can result in disease-free survivors, especially in the young. The outcomes after a second transplantation for patients aged >17 years and for those with lymphoma were especially grim. These data suggest that pediatric patients may be appropriate candidates for a second transplantation. In adults, however, the use of an allogeneic transplantation as salvage therapy after failure of the initial autologous transplantation is generally unsuccessful. Alternative experimental strategies, such as low-dose nonmyeloablative allogeneic minitransplantations, should be considered.