BACKGROUND: Adult T-cell leukemia/lymphoma (ATLL) is an and aggressive mature T-cell neoplasm driven by human T-cell leukemia virus type 1 (HTLV-1) with dismal prognosis. North American ATLL (NA-ATLL) disproportionately impacts individuals of Afro-Caribbean and Latin American descent who face significant disparities in identifying conventional 8/8 HLA-matched donors in international registries. While allogeneic hematopoietic cell transplantation (HCT) is established in East Asian cohorts, outcomes data in Western populations remain limited. We aimed to evaluate post-HCT survival, toxicities, and prognostic risk factors in NA-ATLL. OBJECTIVE: The objective of this study was to evaluate HCT clinical outcomes. Our primary outcome was overall survival (OS), and secondary outcomes included progression-free survival (PFS), non-relapse mortality (NRM), and graft-versus-host disease (GVHD). We tested the hypotheses that alternative donor HCT yields comparable survival to matched donor HCT and assessed variables that may dictate post-transplant survival. STUDY DESIGN: This was a multicenter retrospective registry study evaluating 46 adult patients with NA-ATLL who underwent allogeneic HCT across participating North American centers. Myeloablative conditioning regimens were myeloablative in 21.7%, and alternative donor HCT (without a matched donor) were used in 54.3%. Survival endpoints (OS, PFS) were estimated using the Kaplan-Meier method and compared via log-rank tests. Cumulative incidences of relapse, NRM, acute GVHD, and chronic GVHD were calculated considering competing risks. Independent prognostic variables were evaluated using Cox multivariable proportional hazards regression analysis. RESULTS: There was no significant difference in OS or PFS between those who received matched versus alternative donor HCT (OS p = 0.601, PFS p = 0.376). Patients with acute disease subtype had worse OS compared to lymphomatous (logrank p = 0.043, 3-year OS 22.2% versus 52.3%, respectively). Acute GVHD yielded a 1-year cumulative incidence rate of 45.7% (95%CI; 30.7-59.5%) while that of chronic GVHD was 18.5% (95%CI; 8.5-31.5%). Twenty-seven patients were deceased at time of analysis, primarily due to relapse-related causes in 17 patients. In multivariable analysis, lymphomatous disease subtype (HR 0.3, 95%CI 0.1-0.88, p = 0.027) and Karnofsky performance score of ≥80 (HR 0.26, 95%CI 0.09-0.73, p = 0.01) were significant predictors of OS. CONCLUSION: This is the largest cohort evaluating HCT outcomes in a NA-ATLL population. Alternative donor HCT may expand access to HCT without an apparent increase in non-relapse mortality in this selected cohort. Clinical acute disease subtype shows inferior outcomes compared to lymphomatous subtype. Baseline performance status is a predictor of improved prognosis post-transplant. These data suggest that fit patients should proceed to HCT expeditiously with any donor option available.