Abstract： Background The pathophysiology of late-onset hemorrhagic cystitis (LOHC) is currently not well understood.The aim of this study was to analyze the ailoimmune aetiology in the pathogenesis of LOHC post allogeneic hematopoietic stem cell transplantation (HSCT).Methods A retrospective study was performed on the medical records of 11 patients with immune-related LOHC post allogeneic HSCT. The clinical characteristics, therapy, and outcomes of these patients were analyzed.Results The median time of onset was 42 days after HSCT (range 16-150 days) and the median duration of HC was 43 days (range 29-47 days).All patients presented with prolonged HC for more than 35 days. Nine patients with evidence of cytomegalovirus (CMV) reactivation did not respond to anti-viral therapy even with CMV clearance in the urine post-therapy.Eleven patients with refractory HC received a low dose of corticosteroids and all patients went into complete remission.Conclusion Our data suggest that alloimmune injury is involved in the pathogenesis of HC in at least some patients and that specific therapy might improve the clinical outcome of hemorrhagic cystitis.