Bill Gates said he is giving 50 million US dollars (£38 million) to help fight Alzheimer's disease.
The Microsoft co-founder said the donation to the Dementia Discovery Fund is personal and not through his charitable foundation.
The London-based private fund is backed by government, charities and pharmaceutical firms and seeks new treatments for the progressive, irreversible neurological disease.
In a statement, Mr Gates said men in his family have suffered from Alzheimer's, adding that he is hopeful that in time Alzheimer's could be a chronic condition treatable with medication.
Mr Gates said the first treatments for the disease might not be feasible for a decade or more and would initially be expensive.
He added that the Gates Foundation might consider how to expand access in poorer countries when treatments are developed.
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