Neuropil threads (NT) in the middle temporal gyrus (MTG) were quantitated by computerized image analysis from five patients each with Alzheimer's disease (AD), Pick's disease (PD), and diffuse Lewy body disease (DLBD), four patients with progressive supranuclear palsy (PSP), and five cognitively normal control subjects (24 patients total). All disease groups met clinical and pathological criteria for their respective diseases. The DLBD subjects did not have pathological features of AD. Using the Gallyas silver method, the percentage of cortical area occupied by NT was calculated for each case examined and compiled for each group. Intergroup comparison revealed the percentage of cortical area occupied by NT as follows: AD, 6.87%; PSP, 1.12%; PD, 0.37%; DLBD, 0.04%; control 0.02%. The evaluation disclosed a significance level of p < 0.0001 when AD was compared to control, PD and DLBD cases and a p < 0.001 when compared to PSP. There was no statistically significant difference between control-DLBD, control-PD, control-PSP, DLBD-PSP, PD-PSP, or PD-DLBD cases (p > 0.05). These data indicate the density of neocortical threads is much greater in AD than in other dementing disorders. It also suggests that NT are not related to the intellectual decline in PD, DLBD, and PSP.