Parkinson's disease is caused by the death of a small number of nerve cells that produce a critical chemical called dopamine. The drug L-dopa can partially make up for the lack of dopamine. As time goes on, however, most patients notice that the drugs do not work as well. Oftentimes, patients develop great fluctuations in motor control. Off drugs they cannot move, and on drugs they have excess, exaggerated movements. Research in animals over the last 20 years has shown that dopamine cells can be replaced by transplants of new cells obtained from fetal brain tissue. For the past 14 years, several laboratories around the world have been performing similar transplants of human fetal brain tissue on patients with Parkinson's disease. So far, it has been impossible to compare results from the different groups because no two centers are performing transplants in the same way.