54 patients with idiopathic Parkinson's disease (PD), 26 patients with Alzheimer's disease (AD) and 18 control subjects, all over 55, have performed neuropsychological tests, evaluating global intellectual function (Rosen's cognitive scale, WAIS digit symbol, WAIS similitude and WMS logical memory tests) and visuospatial functions (Rey lacunar pictures, Poppelreuter and Benton line orientation tests). AD group results were distinctly different from those of the PD and the control groups (p less than 0.001). In the PD group, only the Rosen's scale total score and the visuospatial tests were slightly altered (p less than 0.05). In a PD subgroup with a normal Rosen's scale result, the Benton line orientation test was different from controls (p less than 0.05). In another PD subgroup with Rosen's scale score comparable to a midly impaired AD subgroup, all the neuropsychological tests were abnormal. Only the WAIS digit symbol test, altered in this PD subgroup, was different comparing these 2 subgroups (p less than 0.05). With regard to the PD total group, the neuropsychological perturbed PD subgroup was older, had a longer duration of disease, a higher depression's score, less tremor and a worse equilibrium. These results might reflect a neuropsychological defect heterogeneity among PD patients, related to various pathophysiological hypothesis which are discussed in this paper.