BACKGROUND: Epidemiological research on the incidence of traumatic head injuries relies on the correct definition and classification of the injury. OBJECTIVE: To address the use of diagnostic criteria and ICD-10 codes to define minor head injury in Swedish hospitals managing patients with head injury. METHODS: A questionnaire was mailed to all 76 Swedish hospitals managing head injuries. The hospitals were asked what diagnostic criteria they use to define minor head injury, and which ICD-10 codes they use to classify such injuries. RESULTS: 72 hospitals (95%) responded to the survey. The most common criterion was loss of consciousness (76%), followed by post-traumatic amnesia (38%). Almost half the hospitals used other signs and symptoms to define minor head injury. The ICD-10 code S.06 (intracranial injury) was used by 51 of the hospitals (91%). CONCLUSIONS: It is essential that there should be common definitions, classifications, and registration of minor head injuries. The wide variation in definition and classification found in this study emphasises the importance of improved implementation of the present guidelines.