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Few studies have investigated the measurement error of the Mini-Mental State Examination (MMSE) in the same unit of measurement, also known as absolute reliability. This measurement can help determine whether an observed score change for an individual is likely to represent true change. The aim of this study was to investigate the absolute reliability of the MMSE among individuals with dementia that reside in nursing homes. Among 88 participants, 19 (21.6%) were men, 35 (39.8%) had Alzheimer's disease, 35 (39.8%) had vascular dementia, and the mean age was 84.0 years (range 65-98). The participants were tested and retested with the MMSE within 1-6 days. Both tests were administered by the same assessor at the same time of day. The mean MMSE score was 13.7 (range 0-28). The absolute difference between MMSE scores varied from 0 to 6 points, and the differences did not correlate with the corresponding score means (p = 0.874). The smallest detectable change (SDC) between two measurements was 4.00. The SDC was independent of depression, impaired vision and hearing, delirium within the last week, dementia type and age. However, the SDC was 5.56 among men and 3.50 among women (p = 0.003). In conclusion, for individuals with dementia that reside in nursing homes, it seems like their MMSE score needs to change by four or more points between two measurements in order for their score change to be reliably higher than the measurement error.