Taking regular exercise may provide a small benefit for people with early signs of dementia, research suggests.
A clinical trial on two groups of elderly people found those who followed an exercise programme saw a small improvement in overall thinking skills compared to those who did not exercise.
But the results suggested that the benefit may only last as long as people continued with an exercise plan.
The findings, published in the journal Neurology, examined people with early signs of vascular dementia.
This is the second most common type of dementia, after Alzheimer's disease, and affects around 150,000 people in the UK.
It causes memory loss and difficulties with thinking, problem-solving and language - and occurs when there are problems with the supply of blood to the brain.
The new study involved 70 people with an average age of 74.
Half of them took part in a one-hour exercise class three times a week for a six-month period.
The other half were given information on their condition and followed a healthy diet, but were not told to exercise.
Both groups were tested before the study, after it finished and six months later for overall thinking skills, executive function skills - such as planning and organising - and how well they could carry out their daily routine.
The results showed that those who exercised had a small improvement on the test of overall thinking skills - improving by 1.7 points - compared to those who did not exercise.
Study author Teresa Liu-Ambrose, from the University of British Columbia in Vancouver, Canada, said: "This result, while modest, was similar to that seen in previous studies testing the use of drugs for people with vascular cognitive impairment.
"However, the difference was less than what is considered to be the minimal clinically important difference of three points."
Six months after the exercise group stopped taking exercise, their scores were no different than those who did not exercise. There was no improvement on executive function skills or daily activities.
However, the exercise group had better blood pressure control and there were improvements on how far they could walk in six minutes.
Researchers said this is important because high blood pressure increases the risk of dementia.
Dr Rosa Sancho, head of research at Alzheimer's Research UK, said: "We know that regular aerobic exercise can help improve cardiovascular health, but it has also been linked with a healthy brain and a reduced risk of developing dementia.
"This new study suggests that an aerobic exercise programme may be beneficial for people who already have early memory problems.
"While many studies have found a link between physical activity and dementia risk, few have tested specific intervention programmes and it's positive to see new trials in this important area.
"This small trial suggested that benefits may only be apparent while someone is actively engaged in an exercise programme, which will need following up in longer studies.
"With limited treatment options for people with memory decline or dementia, it's important to explore a range of possible therapeutic approaches."
She added: "Physical activity does not necessarily mean running marathons, but can involve a brisk regular walk with friends as part of a normal daily routine for people of any age."
Dr Doug Brown, director of research and development at the Alzheimer's Society, said: "We already know keeping active, along with a balanced diet, is one of the best ways to reduce your risk of dementia.
"This study goes further, suggesting that frequent exercise provides modest improvements in memory and thinking for people who already have vascular dementia.
"Although this was a small study and the benefits of exercise didn't help those involved with daily decision making or activities, it is promising to see researchers focussing on important issues around exercise."