Frontal lobe dysfunction and other cognitive deficits have been described in Parkinson's disease (PD), which may lead to dementia. Both striatal dopaminergic deficiency and regional or global brain volume loss have been suggested to contribute to cognitive decline in PD. We therefore performed a neuropsychological evaluation, structural brain MRI and Fdopa PET in patients with PD and healthy elderly volunteers. PD patients had impaired cognitive performance in many neuropsychological tests compared to controls, not limited just to frontal lobe function tests. Caudate Fdopa correlated positively with performance in verbal (immediate and delayed) and visual memory. Patients with PD showed atrophy in the hippocampus and the prefrontal cortex and hippocampal atrophy was related to impaired memory. Our findings suggest that striatal dopaminergic depletion and global brain volume loss contribute to cognitive impairment in non-demented PD patients, but dysfunction of extra-striatal dopaminergic or non-dopaminergic systems probably plays a role especially in more generalized cognitive impairment.