The possibility of using stem cells to treat Parkinson's disease has excited physicians and patients alike. However, after many encouraging open-label studies of fetal cell transplantation for Parkinson's disease, three randomized, double-blind, placebo-controlled studies found no net benefit. In addition, patients in two of the studies developed dyskinesias that persisted despite reductions in medication. To realize the promise of stem cells, research has been undertaken to understand and overcome the dual problems of unpredictable benefit and troublesome dyskinesias after dopaminergic cell transplantation.