Hemophagocytic lymphohistiocytosis (HLH) is an uncontrolled hyperinflammatory disorder driven by an overactive immune system that results in high mortality. Post-transplant-associated hemophagocytic lymphohistiocytosis (PT-HLH) is a type of secondary HLH that occurs following allogeneic hematopoietic stem cell transplantation (allo-HSCT). The clinical features of PT-HLH remain unclear and diagnostic and prognostic tools have not yet been established. Here, we retrospectively evaluated the clinical manifestations and outcomes of PT-HLH in 94 patients who underwent allo-HSCT. According to our PT-HLH criteria (hyperferritinemia and increased macrophage count in bone marrow), PT-HLH occurred in 12 patients (12.8%). The PT-HLH patients showed splenomegaly ( P  = .001), a higher risk of engraftment failure ( P  = .013), and an increased percentage of macrophages and hemophagocytes in bone marrow aspirates ( P  = .0009 and P  = .0006, respectively). Moreover, univariate and multivariate analyses revealed that the survival rate was lower in PT-HLH patients than non-PT-HLH patients ( P  = .0017 and P  = .034, respectively). This study defines the clinical features of PT-HLH and PT-HLH criteria that could be useful tools for diagnosing PT-HLH.