Researchers studied 87 patients with multiple sclerosis (MS) who were among more than 30,000 people who took part in the 2002 National Health Interview Survey.
The people were divided into three groups: nonsmokers, early smokers who started before age 17, and late smokers who started at 17 or later.
People who started smoking at 17 or later did not show an increased risk of developing multiple sclerosis compared to nonsmokers, according to the researchers.
About 32% of the MS patients were early smokers, compared to 19% of the people who didn't develop the disease, the researchers say.
"Studies show that environmental factors play a prominent role in multiple sclerosis," says study researcher Joseph Finkelstein, MD, PhD, of Johns Hopkins University School of Medicine in Baltimore. "Early smoking is an environmental factor that can be avoided."
The study findings took into account sex, race, age, marital status, education, income level, and region.
The researchers say limited studies in the past also have suggested smoking as a risk factor for MS.
Their findings are to be presented April 25-May 2 at the annual meeting of the American Academy of Neurology in Seattle.