Previous studies have shown altered casein kinase II (CK-II) in Alzheimer's disease (AD). For the present study, the authors analyzed CK-II immunoreactivity at various stages of tangle formation using quantitative laser confocal microscopy and immunoelectron microscopy. AD hippocampal pyramidal cells without neurofibrillary tangles (NFTs) displayed 15% more anti-tau immunoreactivity (P less than 0.01) and 43% more anti-CKII immunolabeling than controls (P less than 0.001). In AD, tangle-bearing hippocampal neurons with strong anti-tau immunoreactivity (threefold increase from controls) showed a significant 22% increase in anti-CKII immunolabeling (P less than 0.01), compared with those without NFTs. Neurons with early neurofibrillary changes showed diffuse anti-CKII immunostaining in their cytoplasm and cell processes. In tangle-bearing neurons, in which a higher level of tau immunoreactivity was detected, anti-CKII immunolabeling was distributed along a fibrillar meshwork in cell bodies and processes. Linear regression analysis of anti-CKII and anti-tau immunoreactivity in AD showed a positive correlation (r = 0.53, P less than 0.001). At the ultrastructural level, anti-CKII was immunolocalized to the paired helical filaments (PHF) of the tangle-bearing neurons, as well as to PHF in neuropil threads and some dystrophic neurites in plaques. These results suggest a possible role for CK-II in tangle formation.