The survival of relapsed acute myeloid leukemia (AML) after autologous hematopoietic stem cell transplantation (Autologous HCT) is very poor. We studied the outcomes of 302 patients who underwent secondary allogeneic hematopoietic cell transplantation (Allo-HCT) from an unrelated donor (URD) using either myeloablative (n=242) or reduced-intensity conditioning regimens (RIC, n=60) reported to CIBMTR. After a median follow-up of 58 months (range 2–160), the probability of treatment-related mortality (TRM) was 44% (95%CI 38–50) at 1-year. The 5-year incidence of relapse and overall survival (OS) was 32% (95%CI 27–38) and 22% (95%CI 18–27), respectively. In multivariate analysis significantly better OS was observed with RIC regimens (Hazard Ratio (HR) 0.51, 95%CI 0.35–0.75, p<0.001), with Karnofsky performance status (KPS) ≥90% (HR 0.62, 95%CI 0.47–0.82, p=0.001) and in CMV-negative recipients (HR 0.64, 95%CI 0.44–0.94, p=0.022). Longer interval (>18 months) from Autologous HCT to URD Allo-HCT was associated with significantly lower Relapse risk (HR 0.19, 95%CI 0.09–0.38, p<0.001) and improved LFS (HR 0.53, 95%CI 0.34–0.84, p=0.006). URD Allo-HCT after Autologous HCT relapse results in 20% long-term leukemia-free survival, with best results with longer interval to secondary URD transplantation, KPS ≥90%, in complete remission, and using RIC regimens. Further efforts to reduce TRM and relapse are still needed.