Neurofibrillary tangles (NFTs), which consist of highly phosphorylated tau, are hallmarks of neurodegenerative diseases including Alzheimer disease (AD). In neurodegenerative diseases, neuronal dysfunction due to neuronal loss and synaptic loss accompanies NFT formation, suggesting that a process associated with NFT formation may be involved in neuronal dysfunction. To clarify the relationship between the tau aggregation process and synapse and neuronal loss, we compared two lines of mice expressing human tau with or without an aggregation-prone P301L mutation. P301L tau transgenic (Tg) mice exhibited neuronal loss and produced sarcosyl-insoluble tau in old age but did not exhibit synaptic loss and memory impairment. By contrast, wild-type tau Tg mice neither exhibited neuronal loss nor produced sarcosyl-insoluble tau but did exhibit synaptic loss and memory impairment. Moreover, P301L tau was less phosphorylated than wild-type tau, suggesting that the tau phosphorylation state is involved in synaptic loss, whereas the tau aggregation state is involved in neuronal loss. Finally, increasing concentrations of insoluble tau aggregates leads to the formation of fibrillar tau, which causes NFTs to form.