Gliosis is the most frequent and therefore important neurocellular reaction to brain insult occurring in diseases ranging from AIDS to infarction. Neuropathological diagnosis of gliosis is based on morphological changes of brain glial cells. Changes commonly agreed to reflect gliosis are qualitative increases in size, number and glial fibrillary acidic protein (GFAP) immunoreactivity of astrocytes. These parameters were morphometrically quantified in brain tissues of 22 individuals who died with 7 diseases and statistically compared to the extent of gliosis independently determined by 3 qualified observers. The data indicate that the extent of gliosis correlated with the increase in size of astrocytes in white matter (p = 0.67) and this relationship was statistically significant (P = 0.0006). In contrast, the extent of gliosis was not correlated with the density of astrocytes nor the intensity of GFAP staining.