Dementia and extrapyramidal signs combine in both Alzheimer's disease (AD) and Parkinson's disease (PD) to produce various degrees of clinical overlap between the two diseases. Rest tremor, positive motor response to dopaminergic drugs, bradyphrenia and disproportionate deficits in visuospatial function, dating capacity, recency discrimination, sequencing and set-shifting are specific features of PD; myoclonus, orofacial dyskinesia, aphasia and rapidly progressive global dementia favours AD. A clearer analysis of the underlying brain-behaviour relationships is necessary to advance our understanding of the origin of cognitive and motor impairment and its treatment.