Current estimates suggest that about one in three patients with Parkinson's disease will become demented. Critical review of the studies on which this conclusion is based indicate that this figure is inflated. Errors in separating idiopathic Parkinson's disease from other causes of the akinetic-rigid syndrome, in differentiating dementia from confusional states, depression, and even normal ageing, and in defining and assessing dementia itself may have led to overestimation of the prevalence. A more conservative estimate is that one in five patients with Parkinson's disease is demented, but this assertion needs to be put to rigorous examination.