Depression sufferers face a significantly higher risk of developing dementia, including Alzheimer's disease, later on in life, US research shows.
Scientists say a link between dementia and depression could be down to lifestyle factors, brain inflammation and the behaviour of some proteins.
The study covered almost 1,000 people with an average age of 79 who were enrolled into a heart study.
At the beginning, all were free of dementia. Psychological tests identified 125, or 13%, of the study participants were classified as being depressed.
By the end of the 17-year study, 164 of the recruits had dementia, of whom 136 were diagnosed with Alzheimer's.
Nearly 22% of those who were depressed at the outset developed dementia, compared with 16.6% of those free from depression. They were around 30% more likely to acquire a dementia illness.
The results, published in the journal Neurology, were unaffected by a person's age, sex, education, or possession of the defective APOE gene that is known to increase the risk of Alzheimer's disease.
Leading researcher Dr Jane Saczynski, from the University of Massachusetts Medical School in Worcester, US, said: "Inflammation of brain tissue that occurs when a person is depressed might contribute to dementia. Certain proteins found in the brain that increase with depression may also increase the risk of developing dementia."