British researchers say smoking "rots" the brain by damaging memory, learning and reasoning.
Researchers at King's College London studied 8800 people over the age of 50 in an investigation into the links between the likelihood of a heart attack or stroke and the state of the brain.
They say their study, published in the journal Age and Ageing, shows high blood pressure and being overweight also seems to affect the brain, but to a lesser extent.
The scientists say people need to be aware that lifestyles can damage the mind as well as the body.
They collected data about the health and lifestyle of their study group and also asked them to perform brain tests, such as making participants learn new words or name as many animals as they could in a minute.
They were all tested again after four and then eight years.
The results showed the overall risk of a heart attack or stroke was "significantly associated with cognitive decline", with those at the highest risk showing the greatest decline.
They also showed a "consistent association" between smoking and lower scores in the tests.