Dementia with Lewy bodies (DLB) is the second commonest cause of dementia in the general population. Several researches have established an association between Down syndrome (DS) and Alzheimer's disease. Very few studies have however showed such an association between dementia with Lewy bodies and Down syndrome. The occurrence of DLB in persons with DS is widely unrecognized. We report the first case of a person who fulfils the operational criteria for DLB and was also found to have Lewy bodies on neuropathological examination. It is important to make an early and accurate diagnosis as patients with DLB may respond differently than Alzheimer's dementia patients to certain behavioural and medical treatments.