A study, published in JAMA Neurology, showed patients with Alzheimer's had four times the levels of DDT lingering in the body than healthy people.
Some countries still use the pesticide to control malaria.
Alzheimer's Research UK said more evidence was needed to prove DDT had a role in dementia.
DDT was a massively successful pesticide, initially used to control malaria at the end of World War Two and then to protect crops in commercial agriculture.
However, there were questions about its impact on human health and wider environmental concerns, particularly for predators.
It was banned in the US in 1972 and in many other countries. But the World Health Organization still recommends using DDT to keep malaria in check.
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