Mid-life heavy smokers have been shown to be at more than double the risk of developing Alzheimer's disease, with those smoking more than two packets of 20 cigarettes daily carrying a 157% greater chance than non-smokers.
A recent study also said that 40-a-day smokers have a 172% increased risk of vascular dementia in the follow-up period.
Greater risk of vascular dementia could be due to the effect of smoking on blood vessels, according to the US researchers who studied the progress of more than 21,000 middle-aged men and women for an average of 23 years.
Lead researcher Dr Rachel Whitmer, from the Kaiser Permanente research institution in Oakland, California, said: "We know smoking compromises the vascular system by affecting blood pressure and elevates blood-clotting factors, and we know vascular health plays a role in risk of Alzheimer's disease."
Participants in the study were enrolled into a dementia survey between 1978 and 1985 when they were 50 to 60 years old.
A total of 1,136 were eventually diagnosed with Alzheimer's and 416 with vascular dementia.
The findings, published in the journal Archives of Internal Medicine, were not affected by race or gender.
Smoking is known to contribute to inflammation and oxidative stress, both of which are believed to be important in the development of Alzheimer's disease.