Co-existent comorbidity is a major determinant of treatment outcome in head and neck cancer. Most of the work pertaining to this topic has been done in the United States, where the standard practice is for trained cancer registrars to grade comorbidity using validated indexes by retrospective notes review. The adult comorbidity evaluation – 27 index (ACE-27) is a validated instrument that has been widely used in head and neck cancer. Although the required clinical data may be available in the notes, a significant amount of historical information is required to grade comorbidity. The aim of this study was to assess the accuracy and inter-rater reliability of the retrospective notes review process, in a typical setting in the United Kingdom (UK), by comparing the information obtained on notes review alone by a physician to that available after a structured patient interview. The study concludes that the retrospective notes review is an accurate and reliable technique for grading comorbidity whose completeness can be improved by the use of patient questionnaire as part of a structured interview.