Objective·To analyze the characteristics of early clinical symptoms of hemorrhagic cystitis (HC) after allogeneic hematopoietic stem cell transplantation (HSCT) and the relationship between the symptoms and disease progression. Methods·The clinical data of 77 children with HC after HSCT, treated in Shanghai Children's Medical Center, Shanghai Jiao Tong University School of Medicine from June 2013 to December 2020, were retrospectively analyzed. Each child was prophylactically applied with mesna in the early stage, and hydration, alkalization, diuretics, anti-infection agents, antispasmodic and analgesic drugs were applied during the course of disease. Moreover, indwelling catheterization and bladder perfusion or cystoscopic electrical coagulation were performed for refractory HC children; Open cystostomy combined with postoperative bladder perfusion was performed when the cystoscope was difficult to perform. HC was graded according to Droller's standard, and the characteristics of urinary irritation symptoms and hematuria symptoms, the onset time of each symptom, the time of progression to severe HC (grade Ⅲ‒ Ⅳ), the duration of disease course, treatment methods and therapeutic effects were recorded. Results·Among the 77 cases, 49 (63.6%) were male and 28 (36.4%) were female. The median age was 7 (5, 11) years old. Sixty-five cases (84.4%) had virus infection during the course of disease, such as BK polyomavirus (BKV), cytomegalovirus (CMV) and Epstein-Barr virus (EBV). Forty-five cases (58.4%) had urinary irritation symptoms as the first symptom of urinary tract, and 32 cases (41.6%) had hematuria. Children who had urinary irritation symptoms as the first symptom eventually developed hematuria symptoms of varying degrees after hydration, alkalization, analgesic drugs or other routine treatments, with a median interval of 2 (1, 5) d, including 3 cases of grade Ⅰ (6.7%), 16 of grade Ⅱ (35.5%), 23 of grade Ⅲ (51.1%), and 3 of grade Ⅳ (6.7%). There were 3 cases of surgical treatment: 1 case of grade Ⅲ was completely relieved after cystoscopic hemostatic treatment, 1 case of grade Ⅳ was partially relieved after open cystostomy combined with postoperative bladder perfusion, and 1 case of grade Ⅳ died for multiple organ failure after cystoscopic hemostatic treatment. Conclusion·More than half of the children with HC after HSCT start with urinary irritation symptoms, and more than half of the children with severe HC also start with it. Such children need more active intervention measures in the early stage. [ABSTRACT FROM AUTHOR]