THE missense point mutation found in the tau gene, which was segregated in a family with frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), has proved to be the causal molecule for widely spread dementia diseases. Here we examined the effects of the tau mutation using confocal analysis. When wild-type tau cDNA was introduced into cells, extensive cell processes and well-developed thick bundles of microtubules were induced. On the other hand, when altered tau cDNA with the mutation (valine337-methionine) was introduced, cell lost processes and microtubule networks resulted in more round cell shape but showed intact mitochondria or endoplasmic reticulum. We conclude that the tau mutation primarily affects the microtubules and resultantly causes the loss of cellular organization and function due to microtubule disruption.