AIMS AND OBJECTIVES: The aim of this study was to describe how nurses document their subjective assessment of the patients' cognitive status in the patients' records and to compare this documentation with an assessment made using a validated evaluation instrument in older patients with a hip fracture. BACKGROUND: There are indications that older people with a hip fracture and impaired cognitive ability do not receive optimal care and that they suffer from a disproportionately high number of complications. Preventing and rapidly detecting confusion is probably an effective strategy for improving care for these patients. To be able to prevent care-related complications and plan for future nursing and medical care, it is necessary to identify patients with impaired cognitive ability. DESIGN: Clinical trial including 362 patients. METHODS: The patients' cognitive function was assessed by a research nurse using a validated instrument, the Short Portable Mental Status Questionnaire, and an independent subjective assessment was made by a ward nurse. The agreement between these assessments was analysed. RESULTS: An assessment of cognitive function by the ward nurse was lacking in 12% of the patients. The assessment made by the nurses did not correspond to the level of orientation according to Short Portable Mental Status Questionnaire in 24% of the patients. In the vast of majority of these cases, the patients were documented as being cognitively alert although they were cognitively impaired according to the Short Portable Mental Status Questionnaire. Among the patients who were cognitively oriented according to the Short Portable Mental Status Questionnaire, the nurses' assessment identified 97% as oriented, but among the patients with impaired cognitive ability according to the Short Portable Mental Status Questionnaire, only 58% were identified as being cognitively impaired by the ward nurses. CONCLUSIONS: An assessment of cognitive function is still lacking in nursing records for a substantial number of older people with a hip fracture and cognitive dysfunction is frequently underdiagnosed in routine health care. RELEVANCE TO CLINICAL PRACTICE: Patient care could be improved if the patients' cognitive function was assessed regularly and objectively by means of a validated evaluation instrument.