The occurrence of dementia in genetic Parkinson's disease is heterogeneous. The onset and progression of diverse forms of familial Parkinson's disease might be different than that of sporadic disease. Since dementia is an age related process, its risk increases with advanced disease severity and duration. The onset and progression of dementia is expected to vary between genetic forms, which present at diverse ages with different symptomatologies. It seems that genetic Parkinson's disease variants in which Lewy bodies are the prominent pathological hallmark - such as in PARK1, PARK4 and PARK8 - dementia is part of the phenotype. On the contrary, in PARK2 which is not accompanied by Lewy body accumulation, patients do not show a systematic cognitive decline. This review presents information on dementia in genetic forms of Parkinson's disease.