The neurite extension factor S100β is overexpressed by activated astrocytes associated with amyloid-containing plaques in Alzheimer's disease, and has been implicated in dystrophic neurite formation in these plaques. This predicts (a) that the appearance of S100β-immunoreactive (S1000β⁺) astrocytes precedes that of dystrophic neurites in diffuse amyloid deposits and (b) that the number of these astrocytes correlates with the degree of dystrophic neurite proliferation in neuritic plaques. As a test of the first prediction, we determined the number of S100β⁺ astrocytes associated with different plaque types: diffuse nonneuritic, diffuse neuritic, dense-core neuritic, and dense-core nonneuritic. Diffuse non-neuritic plaques had small numbers of associated S100β⁺ astrocytes (1.3 ± 0.1 S100β⁺ astrocytes per plaque [mean ± SEM]; 80% of plaques had one or more). These astrocytes were most abundant in diffuse neuritic plaques (4.2 ± 0.2; 100%), were somewhat less numerous in dense-core neuritic plaques (1.6 ± 0.2; 90%), and were only rarely associated with dense-core non-neuritic plaques (0.15 ± 0.05; 12%). As a test of the second prediction, we correlated the number of S100β⁺ astrocytes per plaque with the area of β-amyloid precursor protein (β-APP) immunoreactivity per plaque (an index of the size of the plaques' dystrophic neurite shells) and found a significant positive correlation (r = 0.74, p < 0.001). This correlation was also evident at the tissue level: the numbers of S100β⁺ astrocytes per plaque-rich field correlated with the total area of β-APP immunoreactivity in these fields (r = 0.66, p < 0.05). These correlations support the idea that astrocytic activation and S100β overexpression are involved in the induction and maintenance of dystrophic neurites in amyloid deposits, and support the concept of a glial cytokine-mediated cascade underlying the progression of neuropathological changes in Alzheimer's disease.