Feeling lonely can increase the risk of Alzheimer's in later life, a study suggests.
Researchers who found the link drew a distinction between being alone and loneliness.
The Amsterdam Study of the Elderly (Amstel) looked at risk factors for depression, dementia and high death rates among 2,000 men and women aged 65 and older.
Participants who felt lonely were more than twice as likely to develop Alzheimer's and other forms of dementia over three years as those who did not.
When influential factors including mental and physical health were taken into account, loneliness was still associated with a 64% increased risk of the disease.
But other aspects of social isolation, such as living alone and being widowed, had no impact.
At the start of the Dutch study, 46% of participants were living alone and half were single or no longer married. About one in five, just under 20%, said they felt lonely.
The findings were reported today in the Journal of Neurology, Neurosurgery & Psychiatry.
The authors, led by Dr Tjalling Jan Holwerda, from VU University Medical Centre in Amsterdam, wrote: "These results suggest that feelings of loneliness independently contribute to the risk of dementia in later life.
"Interestingly, the fact that 'feeling lonely' rather than 'being alone' was associated with dementia onset suggests that it is not the objective situation, but, rather, the perceived absence of social attachments that increases the risk of cognitive decline."
The researchers speculated that loneliness may be an effect of early dementia rather than its cause.
"We hypothesise that feelings of loneliness may.. be considered a manifestation of the deteriorating social skills that are seen as part of the personality change accompanying the process of dementia," they wrote.
Another possibility was that loneliness signified extra sensitivity to distress, which is a known risk factor for the development of Alzheimer's disease.
British expert Professor John Bond, from the University of Newcastle, said: "This is not the first paper to be published on this subject.
"More research is needed to determine whether it is a risk factor or in fact an early symptom. There is strong evidence to suggest that the best way to reduce your risk of dementia is by regularly exercising, eating a diet rich in fruit and vegetables and not smoking."