More than 200 patients with Parkinson's disease (PD) have received intrastriatal grafts of human embryonic mesencephalic tissue. The clinical trials demonstrate that grafted dopamine (DA) neurons can survive in the human parkinsonian brain and reinnervate part of the host striatum. Long-term graft survival and function, at least up to 6 years after transplantation, is possible in PD despite a progressive degeneration of the patient's own DA neurons. A majority of patients with surviving grafts show long-term improvement of therapeutic value, but symptomatic relief is incomplete. Current research strategies to develop neural transplantation as a treatment for PD include (i) to increase DA neuron survival and density and extent of the dopaminergic reinnervation in the striatum; (ii) to implant DA neurons in denervated regions outside the caudate-putamen and to reconstruct the nigrostriatal pathway; and (iii) to find other sources of cells suitable for grafting.