Smoking more than doubles the risk of developing dementia and Alzheimer's disease, according to a study published Friday that is hailed as the largest effort to investigate a link.

Although earlier studies on the relationship between smoking and Alzheimer's have conflicted, scientists say the latest findings are important because of the study's size and because it was the first major project to evaluate people before they had developed brain disease.The study by researchers at Erasmus University in the Netherlands, published in this week's issue of the British medical journal The Lancet, followed 6,870 men and women aged 55 and older living in a suburb of Rotterdam.

It found that smokers were 2.2 times more likely to develop dementia of any kind and had a risk for Alzheimer's disease that was 2.3 times higher than those who had never smoked cigarettes. Former smokers had a slightly higher risk than life-long abstainers, but not significantly so.

Alzheimer's disease, characterized by the degeneration of brain cells, is the most common form of dementia. It is estimated to affect nearly 18 million people worldwide, or 3 percent of people over the age of 60, according to the not-for-profit organization Alzheimer's Disease International.

Some earlier studies found that smoking increases the risk of developing Alzheimer's disease, while others concluded it protects against it.

Unlike previous studies, none of the people in the Rotterdam study had dementia when first examined. They were asked about their smoking habits and divided into smokers, former smokers and those who had never smoked. Two years later, 146 of them had developed dementia and 105 of those had been diagnosed with Alzheimer's disease.

Scientists are unsure exactly how smoking might contribute to Alzheimer's.

The study did not give the number of people affected in each group because the risk of developing dementia grows as people get older and giving raw figures for each of the three groups would have made the results biased, researchers said.

The risk was calculated by taking all factors into account, including age, sex, education and alcohol intake.

Within a random sample of the entire group, the researchers also examined the effect of smoking on people who carried a gene believed to increase the risk of Alzheimer's disease. They found that despite the overall result that smoking doubled the risk of developing the degenerative disease, smoking did not increase the risk of developing Alzheimer's among those who carried the gene.

But smokers who did not have the gene were found to be four times more likely to develop the disease, the study said.