Several recent studies have shown that dementia is common in Parkinson's disease (PD), and that in some patients, cognitive impairment occurs even at the time of diagnosis. The point prevalence of dementia in PD is close to 30% and the incidence rate is increased 4-6 times as compared to controls. The cumulative prevalence is very high, at least 75% of PD patients who survive for more than 10 years will develop dementia. The mean time from onset of PD to dementia is approximately 10 years. However, there are considerable variations, and some patients develop dementia early in the disease course. Earlier onset of dementia is associated with more structural brain changes. The most established risk factors for early dementia are old age, severity of motor symptoms, in particular postural and gait disturbances, mild cognitive impairment and visual hallucinations. The genetic contributions to dementia are currently not clear and need to be explored in future studies.