To assess the role of microglial cells in senile plaque (SP) formation, we examined the density and distribution of microglia in the temporal neocortex of three groups of nondemented individuals, chosen to represent sequential stages of SP formation (no SP, n = 14; diffuse plaques (DP) only, n = 12; both DP and neuritic plaques (NP), n = 14) and patients with Alzheimer's disease (AD, n = 11). The mean density of microglia was significantly greater in the AD group. In nondemented individuals, the presence of NP but not DP was associated with an increased number of microglial cells. Most NP (91 %) were focally associated with microglial cells. DP less commonly contained microglia, however, individuals with some NP had microglia within a greater proportion of their DP (47%) than did those with only DP (19%). These findings suggest that: (a) microglia are not involved in the formation of DP; (b) the presence of NP is associated with both an overall increase in microglia and the focal aggregation of cells around NP; (c) microglia may be locally involved in the conversion of DP into NP. This final point represents the most significant aspect of this study, providing the first quantitative evidence to support a specific role for microglia in the formation of NP from DP.