The densities of neurofibrillary tangles (NFT) and neuritic plaques (NP) were assessed quantitatively in the brains of 303 mentally retarded adults 23 to 90 years of age at the time of their deaths (mean = 59.5 years). Cases with Down's syndrome, hydrocephalus and metabolic disorders were excluded from the study. Examinations of frontal, temporal, parietal, and occipital cortex, as well as hippocampus and parahippocampal gyrus were made in every case. NPs and/or NFTs were observed within the brains of 163 cases (53.8%). Detailed analyses indicated that NP density within all brain regions examined was positively related to age, with the largest age associated increases in density seen in frontal and temporal regions. In contrast, NFT density increased with age only within hippocampus and parahippocampal gyrus, but not neocortex. In addition, NP lesions within neocortex were more diffusely distributed across regions for older compared to younger cases, while no similar age-associated change in the topography of NFTs was observed. Finally, factor analyses of the combined NP and NFT data indicated that, while strong correlations existed across the various brain regions for measures of NP and NFT densities, considered separately, there was virtually no indication of regional associations between these two types of lesions. While these data, from cases with mental retardation, cannot be generalized directly to the nonretarded population, they provide strong evidence that models of Alzheimer pathogenesis must take into account the fact that regional densities of NPs and NFTs, and, therefore, the underlying processes associated with formation of these lesions, can be largely independent.