While a fitness regime improved physical fitness in people with mild to moderate dementia, it "does not slow cognitive impairment," researchers reported in The BMJ medical journal.
It is generally accepted that exercise can delay the onset of Alzheimer's and other forms of dementia.
But whether or not it can slow symptoms after the onset of mental decline, has been the subject of much debate.
For the latest study, researchers took 494 people in England who had been diagnosed with dementia, and assigned 329 of them to an exercise programme.
They took part in 60-90-minute group sessions in a gym twice a week for four months, and home exercises for an additional hour per week.
The average age of the group was 77.
Participants were assessed at six and 12 months after starting the programme.
The researchers noted that cognition had declined in both the exercise and non-exercise groups.
In the exercise group, the decline was steeper, "however, the average difference was small and clinical relevance was uncertain," said a press statement.