There may be a link between a rare blood type and memory loss in later life, American research suggests.
People with AB blood, found in 4% of the population, appear more likely to develop thinking and memory problems than those with other blood groups.
The study, published in Neurology, builds on previous research showing blood type may influence heart risk.
A charity said the best way to keep the brain healthy was a balanced diet, regular exercise and not smoking.
A US team led by Dr Mary Cushman, of the University of Vermont College of Medicine, Burlington, analysed data from about 30,000 US citizens aged 45 and above.
It identified 495 participants who had developed thinking and memory problems, or cognitive impairment, during the three-year study.
They were compared to 587 people with no cognitive problems.
People with AB blood type made up 6% of the group who developed cognitive impairment, which is higher than the 4% found in the general population.
They were 82% more likely to have difficulties with day-to-day memory, language and attention, which can signal the onset of dementia.
However, the study did not look at the risk of dementia.
The study supported the idea that having a certain blood group, such as O, may give a lower risk for cardiovascular disease, which in turn protected the brain, the researchers said.
"Our study looks at blood type and risk of cognitive impairment, but several studies have shown that factors such as high blood pressure, high cholesterol and diabetes increase the risk of cognitive impairment and dementia," said Dr Cushman.
"Blood type is also related to other vascular conditions like stroke, so the findings highlight the connections between vascular issues and brain health. More research is needed to confirm these results."
Commenting on the study, Dr Simon Ridley, head of research at Alzheimer's Research UK, said the research did not look at risk of dementia, and it was too early to say whether AB blood group might be linked to a higher risk of the condition.
"Current evidence suggests the best ways to keep the brain healthy are a balanced diet, not smoking and regular exercise," he said.