Allogeneic hematopoietic stem cell transplantation (allo-HSCT) carries the risk of immune-related complications, including rare alloimmune phenomena. Type 1 diabetes (T1D), a chronic disease marked by pancreatic beta-cell destruction, is rarely observed after allo-HSCT, and its pathogenesis in this setting remains unclear. We report two pediatric cases of T1D that developed several years after allo-HSCT for primary immunodeficiencies and review clinical and HLA data. A review of relevant literature was also conducted to put these findings in perspective. Both patients developed T1D 6.5 to 9.5 years after allo-HSCT. In both cases, pancreatic autoantibodies were detected. Neither donor had a history of autoimmune disease. Literature review suggests that post-HSCT T1D may arise through alloimmune mechanisms rather than a manifestation of graft-versus-host disease. Improving outcomes may depends on close multidisciplinary follow-up. Development of screening protocols and early detection of potential immunological processes are required.