Parkinson's disease has been thought to be a good disease model where cell replacement could actually help. It's caused by the damage of a particular kind of neural cells by an abnormally formated protein. Therefore anything replacing these damaged cells are thought to be effective. Stem cell researchers around the world are trying to make this kind of cells from embryonic stem cells for replacement. However it should be noted that earlier trials in 1990s already started injecting normal cells from aborted fetuses into the affected patients' brains to replace those cells destroyed. Although not exactly same as stem cells, these researches did offer some useful information. First, the injected cells did actually survive and incorporated to the patients' brain system. Second, these cells could offer some benefit (although not dramatic in most cases).
Now, some of the patients have died and postmortem examinations told us something surprising -- in some cases, some of the injected cells acquired similar damage as the original disease cells. Parkinson's disease usually only hits old people. However those injected cells are still relatively "young" (slightly over one decade), how did these cells got the dangerous protein? No one could answer yet. Although only 6 cases were analyzed, these findings indicate the complex of this disease. Will the same thing happen to stem cell derived cells in the future? How to get away with this problem?
Further research is necessary. problems will be solved eventually.