Women are more likely than men to develop Alzheimer's, and taking HRT increases the risk by a further 19 per cent.
The latest research turns on its head earlier studies that had thought HRT had a protective effect against Alzheimer's and dementia because it kept levels of estrogen higher.
Researchers from the University of Helsinki in Finland analysed files on more than 84,000 Finnish women who were diagnosed with Alzheimer's between 1999 and 2013 and compared them to a similar number of non-Alzheimer's women, before assessing medical histories, including the use of HRT.
They discovered that women taking estradiol-only HRT had a slightly lower risk of 9 per cent, but it rose to 17 per cent for those taking the more common estrogen-progestogen HRT preparations.
The researchers think that maintaining levels of estradiol beyond the menopause could be a factor in the development of Alzheimer's.
They point out that their research merely shows an association, and this doesn't prove a cause, although compounds from HRT have been seen in brain plaques that are common in Alzheimer's patients.