We studied regional gray matter density in the hippocampus in Parkinson's disease (PD) patients. We obtained magnetic resonance scans in 44 PD patients (PD patients with dementia (PDD) = 9, non-demented PD patients with visual hallucinations (PD + VH) = 16, and PD patients without dementia and without visual hallucinations (PD - VH) = 19) and 56 controls matched for age and years of education. A region of interest (ROI) of the hippocampus following voxel-based morphometry (VBM) procedures was used to perform group comparisons, single-case individual analysis and correlations with learning scores. Group comparisons showed that PDD patients and PD+VH patients had significant hippocampal gray matter loss compared to controls. In PDD patients, hippocampal gray matter loss involved the entire hippocampus and in PD+VH this reduction was mainly confined to the hippocampal head. 78 % of PDD patients, 31 % of PD+VH patients and 26 % of PD-VH patients had hippocampal head gray matter loss when compared to controls. These results suggest that in PD the neurodegenerative process in the hippocampus starts in the head of this structure and later spreads to the tail and that, in addition, memory impairment assessed by Rey's Auditory Verbal Learning Test (RAVLT) correlates with hippocampal head gray matter loss.