Parkinsonism-dementia together with amyotrophic lateral sclerosis is a highly prevalent disorder among the native Chamorro population of Guam and is accompanied by severe widespread neurofibrillary tangle formation. In the present study we compared the regional and laminar distribution of neurofibrillary tangles in the cerebral cortex of 5 Guamanian parkinsonism-dementia cases to 9 Caucasian Alzheimer's disease cases. Although in both diseases the superior frontal and inferior temporal cortex were affected to a comparable degree, there was a striking difference in the laminar distribution of neurofibrillary tangles. Neurofibrillary tangles in Alzheimer's disease are known to be more numerous in layers V-VI than in layers II-III of frontal and temporal cortex, however in the Guam cases, the opposite distribution was observed with most of the tangles located within layer II and the superior part of layer III and relatively low tangle density in layers V-VI. Interestingly, in both conditions, the hippocampal pyramidal layer showed a comparable degree of degeneration. Moreover no amyloid deposits and neuritic plaques were observed in the Guam brains, whereas they were frequent in the Alzheimer's disease cases. Previous studies have suggested that the clinical symptomatology observed in patients suffering from Alzheimer's disease is related to the dramatic loss of specific corticocortically projecting neurons in the neocortex. The present data on Guam parkinsonism-dementia further support this hypothesis, although the set of corticocortical connections affected in Guam cases might differ from that observed in Alzheimer's disease, which may contribute to some extent to the different clinical symptoms of the dementia observed in Guamanian patients.