© 2018 American College of Physicians. Background: The multifactorial mechanisms associated with radical reductions in HIV-1 reservoirs after allogeneic hematopoietic stem cell transplant (allo-HSCT), including a case of HIV cure, are not fully understood. Objective: To investigate the mechanism of HIV-1 eradication associated with allo-HSCT. Design: Nested case series within the IciStem observational cohort. Setting: Multicenter European study. Participants: 6 HIV-infected, antiretroviral-treated participants who survived more than 2 years after allo-HSCT with CCR5 wildtype donor cells. Measurements: HIV DNA analysis, HIV RNA analysis, and quantitative viral outgrowth assay were performed in blood, and HIV DNA was also measured in lymph nodes, ilea, bone marrow, and cerebrospinal fluid. A humanized mouse model was used for in vivo detection of the replication-competent blood cell reservoir. HIV-specific antibodies were measured in plasma. Results: Analysis of the viral reservoir showed that 5 of 6 participants had full donor chimera in T cells within the first year after transplant, undetectable proviral HIV DNA in blood and tissue, and undetectable replication-competent virus (<0.006 infectious unit per million cells). The only participant with detectable virus received cord blood stem cells with an antithymocyte globulin- containing conditioning regimen, did not develop graft-versushost disease, and had delayed complete standard chimerism in T cells (18 months) with mixed ultrasensitive chimera. Adoptive transfer of peripheral CD4+ T cells to immunosuppressed mice resulted in no viral rebound. HIV antibody levels decreased over time, with 1 case of seroreversion. Limitation: Few participants. Conclusion: Allo-HSCT resulted in a profound long-term reduction in the HIV reservoir. Such factors as stem cell source, conditioning, and a possible "graft-versus-HIV-reservoir" effect may have contributed. Understanding the mechanisms involved in HIV eradication after allo-HSCT can enable design of new curative strategies.