Background: Numerous studies have documented that persons with Downs syndrome (DS) are at an increased risk of Alzheimers disease (AD). However, at present it is still not clear whether or not all persons with DS will develop dementia as they reach old age. Methods: We studied 506 people with DS, aged 45 years and above. A standardized assessment of cognitive, functional and physical status was repeated annually. If deterioration occurred, the patients were examined and the differential diagnosis of dementia was made according to the revised Dutch consensus protocol and according to the ICD-10 Symptom Checklist for Mental Disorders. We compared our findings with those reported in the literature. Results: The overall prevalence of dementia was 16.8. Up to the age of 60, the prevalence of dementia doubled with each 5-year interval. Up to the age of 49, the prevalence is 8.9, from 50 to 54, it is 17.7, and from 55 to 59, it is 32.1. In the age category of 60 and above, there is a small decrease in prevalence of dementia to 25.6. The lack of increase after the age of 60 may be explained by the increased mortality among elderly demented DS patients (44.4) in comparison with non-demented patients (10.7) who we observed during a 3.3-year follow-up. There was no decrease in incidence of dementia in the age group of 60 and above. Our findings are very similar to those published in the literature. Patients with dementia were more frequently treated with antiepileptic, antipsychotic and antidepressant drugs. The history of depression was strongly associated with dementia. Conclusions Our study is one of the largest population-based studies to date. We found that despite the exponential increase in prevalence with age, the prevalence of dementia in the oldest persons with DS was not higher than 25.6.