The study included 2,267 men of Japanese ancestry living in Hawaii and enrolled in a long-term health study.
The men took a smell test in the early to mid-1990s, when they were nearly 80 years old, on average. None had Parkinson's disease at the time.
Taking the smell test involved smelling 12 different odors, one at a time, and correctly identifying them. The 12 odors were banana, chocolate, cinnamon, gasoline, lemon, onion, paint thinner, pineapple, rose, soap, smoke, and turpentine. The men got one point for every correct answer.
Over the next eight years, 35 men were diagnosed with Parkinson's disease. Those men tended to have low scores on the smell test four years before their Parkinson's diagnosis.
Other factors -- including age and past smoking habits -- didn't affect the results.
"We found that impaired olfaction is associated with an increased risk of Parkinson's disease within four years," write G. Webster Ross, MD, ofHonolulu's Veterans Affairs Pacific Islands Health Care System, and colleagues.
It's not clear how the sense of smell and Parkinson's disease are related, or if the findings apply to women. And the findings don't mean that all older men who have trouble identifying odors will develop Parkinson's disease.
The study appears in the February 2008 edition of the Annals of Neurology.