Oral mucositis is a common complication in patients undergoing hematopoietic stem cell transplantation. Accurate oral mucositis grading is essential for both clinical practice and oral mucositis research. This study aimed to evaluate the accuracy of daily oral mucositis grading by nurses in a tertiary hospital in Australia. A retrospective study was undertaken to review the daily patient oral assessment record, including diet, pain, erythema, ulceration and the oral mucositis grade based on World Health Organization (WHO) oral mucositis grading scale. The accuracy of the grade was determined by the observations recorded, and reasons for inaccuracy were documented. Any repetition of the same error in the same patient was noted. In total, 6841 oral assessments in 373 patients, conducted between 2017 and 2020, were reviewed. A total of 70% (N = 4781) were graded correctly. Of these, 64% (N = 3043) were grade 0. When the grade 0 scores were excluded, the accuracy of grading was reduced to 46% (N = 1738). Common reasons for incorrect grading included: unable to grade due to diet not specified, no ulceration and no pain was scored grade 1, no ulceration was scored as grade 2-4, oral intake was not taken into account, and pain without ulcer was scored 0. A total of 77% of the errors were repeated in the same patient on consecutive days. Our results suggest there is frequent inaccurate evaluation of oral mucositis and a need for nurse training to accurately assess oral mucositis. © 2021. Crown.