Parkinson's disease patients have double the risk of developing potentially lethal melanoma, government researchers reported Tuesday. Researchers have long suspected such a link, but the new study, reported in the journal Neurology, provides the strongest evidence to date.

Researchers are at a loss to explain how the link occurs biologically, but they suspect it may be a combination of environmental exposure and genetic predisposition. The association is particularly strange, experts said, because Parkinson's patients, in general, have a below-normal risk of developing most types of cancer.

Establishing a link between Parkinson's and melanoma is difficult because both are relatively rare diseases. About 1 million Americans have a diagnosis of Parkinson's, and about 70,000 Americans are diagnosed with melanoma each year. The number with both diseases is therefore relatively small. To increase the chances of finding statistically significant results, Dr. Honglei Chen, a neuroscientist at the National Institute of Environmental Health Sciences in Research Triangle Park, N.C., and colleagues combined the results from 12 studies conducted between 1965 and 2010, a process known as a meta-analysis. Most of the studies had fewer than 10 patients with both conditions, but combining them yielded a number large enough for statistical significance.