Researchers said self-rated health could be a simple tool to determine health risk.
Dr Christophe Tzourio and colleagues studied 8,169 people aged over 65 years who were living in the community.
At the start of the study, participants were asked to rate their own health. They were then assessed an average of 6.7 years later and 618 were found to have developed dementia.
Compared with those who rated their health as good, those rating their health as poor had a 70 per cent higher risk of developing dementia. Those rating their health as fair had a 34 per cent higher risk of developing dementia.
The researchers found that Alzheimer's disease and vascular dementia were both associated with poor self-rated health, though the association was stronger for vascular dementia. However, self-rated health was a stronger predictor of dementia in people without cognitive complaints or functional disability.
The researchers said: 'Our data suggest that a simple self-rated health question could raise awareness about a patient's risk of dementia, particularly in those without conditions that are indicative of possible cognitive difficulties, such as cognitive complaints or disabilities.'
The researchers said studies should look at interventions for managing patients with poor self-rated health, such as controlling vascular risk and stimulating cognitive abilities.