Researchers in Finland, Sweden and at Kaiser Permanente in Oakland followed more than 21,000 middle-aged people who smoked two packs a day for more than 20 years.
The heavy smokers had a 157 percent higher risk of developing Alzheimer's and a 172 percent higher risk of developing vascular dementia -- another common form of dementia.
Researchers think smoking could cause dementia by narrowing blood vessels in the brain.
