Sensitive silver methods for extracellular amyloid and intraneuronal cytoskeleton abnormalities (neurofibrillary tangles and neuropil threads) were employed to examine the cortical pathology in Parkinson's disease. In cases with cognitive impairment many plaque-like amyloid deposits were found in the cerebral cortex. Neuritic plaques were rare or absent. Neither the Ammon's horn nor the isocortex revealed a sufficiently large number of tangles to permit the diagnosis of a coexisting fully developed Alzheimer's disease. Large numbers of neurofibrillary tangles and neuropil threads were only found in layer Pre-alpha of the entorhinal cortex. This layer gives rise to major portions of the perforant tract, a pathway which serves as a link in the transmission of data from isocortical association areas to the hippocampal formation. During the course of Parkinson's disease the hippocampal formation is thus endangered to become disrupted from isocortical influences. It is concluded that the cognitive impairment shown by many individuals suffering from Parkinson's disease may partly be caused by cortical lesions.