The authors evaluated the prevalence of dementia in centenarians. In this population-based survey, persons living in Denmark who turned 100 during the period April 1, 1995--May 31, 1996 (N = 276) were interviewed and examined at their residences. Additional health information was retrieved from medical files, including the National Discharge Registry. A participation rate was 75%, and no differences were found between participants and nonparticipants regarding sex and type of housing. The prevalence of mild to severe dementia in centenarians was 51%; 37% had no signs of dementia. Among the 105 demented centenarians, 13 (12%) had diseases (vitamin B12 and folic acid deficiencies, hypothyroidism, Parkinson's disease) that could contribute to a dementia diagnosis. Of the remaining 92 demented participants, 46 (50%) had 1 one or more cerebro- or cardiovascular diseases known to be risk factors in the development of dementia. The prevalence of these risk factors was the same in demented and nondemented participants, whereas hypertension was significantly more frequent in nondemented than demented participants. Dementia is common but not inevitable in centenarians. Cerebro- and cardiovascular diseases are equally common in demented and nondemented persons.