This study attempted to determine how many patients admitted with a head injury do not have the diagnosis recorded in the medical notes, and the factors associated with this failure. This observational study took place over four weeks with a review of notes of all patients in the trauma service admission wards in a District and Teaching hospital. All patients aged 16 years to 65 years admitted to the inpatient trauma wards were included, and the frequency of diagnosis of head injury made by the investigator was compared with the frequency of a recorded diagnosis of head injury in the notes. Of 107 patients admitted 47 had had a head injury; 24 did not have the diagnosis recorded, and four of these had moderate or severe injuries. A failure to record diagnosis was more likely in the presence of other more severe injuries (21/28), and in patients with minor or trivial injuries (20/30). We conclude that head injury registers are likely to miss a significant number of patients admitted to hospital unless specific attempts are made to identify and record the diagnosis.