Background:Oral mucositis (OM) is a painful and debilitating complication of allogeneic hematopoietic stem cell transplantation (HSCT). Conditioning regimen correlates development of OM, but no studies have analyzed incidence and severity of OM among various condition regimens.Aims:This study was designed to evaluate the incidence and severity of OM receiving various conditioning regimens in early period after HSCT.Methods:We included 120 adult patients with a median age of 54.5 years (range 18 ‐ 72) who received allogeneic HSCT at Kindai University Hospital. Four conditioning regimens were administered for patients before transplantation based on the underlying diseases. Twenty‐two patients were received cyclophosphamide (CY)/total body irradiation (TBI) regimen (CY/TBI), 27 were received busulfan (BU)/CY regimen (BU/CY), 54 were received fludarabine (FLU)/BU regimen (FLU/BU), and 17 were received FLU/ melphalan (MEL) regimen (FLU/MEL). National Cancer Institute Common Toxicity Criteria for Adverse Events scale was used to assess the severity of OM. Severe OM defined as grade 2 to 4.Results:OM of any grades occurred 77.3% of CY/TBI, 48.1% of BU/CY, 85.2% of FLU/BU, and 70.6% of FLU/MEL. There were significant differences between CY/TBI and BU/CY (p = 0.05), and BU/CY and FLU/BU (p = 0.001). There was no patient with grade 4 OM. The incidence of severe OM (grade 2 ‐ 3) was 54.5% of CY/TBI, 18.5% of BU/CY, 57.4% of FLU/BU, and 52.9% of FLU/MEL. There were significant differences between CY/TBI and BU/CY (p = 0.02), BU/CY and FLU/BU (p = 0.0009), and BU/CY and FLU/MEL (p = 0.02).Summary/Conclusion:Incidence and severity of OM in patients receiving BU/CY regimen was the lowest among 4 types conditioning regimens.