The goal of this clinical trial is to test the assess the diagnostic effectiveness of endocytoscopy in patients with underlying UGI aGvHD. The main question it aims to answer is: the effectiveness of ECS for diagnosing UGI aGvHD. Participants will undergo endoscopy, endocytoscopic evaluation and biopsy simultaneously. If there is a comparison group: Researchers will compare endoscopic, endocytoscopic and pathologic findings of these patients.

Inclusion Criteria:

• aged ≥ 14 years
• developed UGI symptoms within first three months after engraftment or during the tapering of immunosuppressive agents

Exclusion Criteria:

• < 14 year
• with underlying gastrointestinal disease before allo-HSCT
• incomplete medical information

Patients who underwent first allogeneic hematopoietic stem cell transplantation at Ruijin Hospital and developed persistent upper gastrointestinal symptoms after neutrophil engraftment were screened. Eligible participants were those with suspected upper gastrointestinal acute graft-versus-host disease whose symptoms did not resolve after at least 72 hours of supportive care and who underwent conventional endoscopy combined with endocytoscopy.