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WASHINGTON — Rats with the type of brain-cell loss found in Parkinson's disease were cured within weeks by transplants of neurons made from embryonic stem cells, scientists report in the online journal Nature Medicine.

The finding was tempered by the discovery that the transplants gave rise to noncancerous tumors, according to the report by a team at Cornell University and the University of Rochester.

"We were hoping for the best result and we got it," Steven Goldman, a co-author of the study, said Sunday. "We were fearing the downsides and we got those, too."

Scientists have tried to cure Parkinson's with stem cells before. Their success has been limited because they could only convert about 20 percent of all cells in a culture into the dopamine-generating neurons that could replace those lost to the degenerative brain disease.

In the Cornell-University of Rochester experiment, scientists used cells derived from the brain tissue of miscarried fetuses. That allowed them to create a more concentrated collection of dopamine-making neurons than in previous efforts, Goldman said.

"The animals got all better," he said.

The tumors found in the rats may have formed from stem cells that hadn't become neurons, Goldman said. Neurons don't normally divide.

The tumors may have been the result of the age of the stem-cell line used in the experiment, Goldman said. Labs funded by the U.S. Department of Health and Human Services can use only certain stem cells, which come from embryos obtained years ago.

While newer cells might create fewer tumors, "We don't know it and can't know it" under the current policy, he said.

Parkinson's disease, with tremors and impaired balance, afflicts at least 500,000 Americans.