BackgroundDiabetes mellitus is associated with cognitive dysfunction, but it is not clear whether the disorder increases the risk of conversion from mild cognitive impairment to dementia.AimsTo determine the association between diabetes mellitus and dementia conversion in people with mild cognitive impairment (Peterson's criteria) in a prospective community-based study.MethodPeople over 65 years old were approached through primary care practices in south London, UK, and those with mild cognitive impairment (n = 103) were followed up for 4 years. Presence of diabetes was established from self-report and information from general practitioners.ResultsNineteen participants progressed to dementia, with the predominant diagnosis being probable or possible Alzheimer's disease (in 84%). Only diabetes mellitus was associated with progression to dementia (hazard ratio 2.9, 95% CI 1.1–7.3) after adjustment for sociodemographic factors, APOE4, premorbid IQ and other health conditions.ConclusionsDiabetes mellitus increases not only the risks of dementia and mild cognitive impairment but also the risk of progression from such impairment to dementia.