In fact, the treatment demonstrated serious side effects in 15 percent of the 40 patients, including uncontrollable writhing and jerking, the study published in Thursday's New England Journal of Medicine showed.
While the results are considered a setback for Parkinson's research, it's not clear whether this study will impact the wider use of stem cells, which many scientists believe may help treat Parkinson's, Alzheimer's and even spinal cord injuries.
The debate isn't about the success or failure of the research, it's about where the stem cells originate, reports CBS News Correspondent Wyatt Andrews. The most effective stem cells, researchers believe, come from human embryos either from abortions or leftover from in vitro fertilization.
Scientists are focused on the controversial research because they have found that stem cells can, in theory, grow to replace any human cell.
"For the first time, we have in culture in a lab a human cell that has the ability to form any cell in the body," said Dr. John Gearhart of Johns Hopkins.
But while in the Parkinson's study the implanted stem cells survived and grew into the right kind of brain cells, they did not help patients older than 60.
Younger patients who make up about 40 percent of the 60,000 people diagnosed each year in the United States improved a bit, but only for a year.
After that, the cells apparently did their job too well in some patients, causing excess movements because they produced more of the needed nerve transmitter dopamine than the body could use.
"There was tremendous hope that stem cell therapy could be a cure," said neurologist Dr. J. William Langston, founder of the Parkinson's Institute in Sunnyvale, Calif. "This study really points out the problems we have to solve before that can happen."
But the research faces a big political test next week when the Bush administration decides whether to allow federal money to be used for the controversial research.
There's powerful opposition to using human embryos for research from anti-abortion activists, who see it as murder.
"Whether or not advances could be made from stem cells obtained from embryonic human persons becomes irrelevant, because it is never OK," said Cathy Brown of the American Life League. "It is never justifiable to take the life of another human person."
On the other side is a growing lobby of scientists, including 80 Nobel laureates, who argue in a letter to the president that not using embryos that might be discarded anyway would be unconscionable.
Parkinson's is a progressie brain disease marked by tremors, stiffness, slowness and loss of balance. The symptoms grow as the brain loses cells that produce dopamine, a transmitter that carries messages to the nerve cells controlling motion.
The disease afflicts more than a million American's, among them former Attorney General Janet Reno and actor Michael J. Fox.
Steve Moritz, who suffers not from Parkinson's but from multiple sclerosis, has pinned his hopes on stem cell research.
His disease makes it a struggle to play with his children or climb the stairs in his home, and he can't understand why anyone would oppose using stem cells to cure this degenerative disease.
"There is research out there that will enable me able to walk my twin daughters down the aisle someday," Moritz said.
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