BACKGROUND: cognitive impairment is an important part of the diagnostic criteria for dementia. The Mini-Mental State Examination (MMSE) is recommended to test for cognitive impairment and to monitor medication response. OBJECTIVES: we examined the prevalence of cognitive impairment in the UK and assessed associations with cognitive impairment. DESIGN: cross-sectional survey as part of a cluster randomised trial. SUBJECTS: representative sample of people aged 75 years and over. METHODS: all subjects had a detailed baseline health assessment including the MMSE. RESULTS: a total of 15,051 subjects completed the assessment (71.9%). Almost two-thirds of subjects were female (61.5%) and almost half were aged between 75 and 79 years (47.0%). The prevalence of cognitive impairment was 18.3% (95% confidence intervals (CI) = 16.0-20.9) at a cut-off of 23/24, and 3.3% (95% CI = 2.8-4.0) at 17/18. Those with impairment (MMSE 23/24) were significantly more likely to have hearing (odds ratio (OR) 1.7), vision (OR 1.7) and urinary incontinence problems (OR 1.3), have two or more falls in the previous 6 months (OR 1.4), and report poorer health (OR 1.9). Almost half the participants lived alone (n = 7,073; 47.0%) and of these almost one-fifth were impaired (MMSE 23/24; 19.4%). CONCLUSIONS: there was a high prevalence of cognitive impairment. This representative sample demonstrates the potential burden of disease and service demands. It supports the need for a broader assessment of functioning as recommended by the National Service Framework for Older People, particularly in people with cognitive impairment.