BACKGROUND. Trials are under way to determine whether fetal nigral grafts can improve motor function in patients with Parkinson's disease. Some studies use fluorodopa uptake on positron-emission tomography (PET) as a marker of graft viability, but fluorodopa uptake does not distinguish between host and grafted neurons. There has been no direct evidence that grafts of fetal tissue can survive and innervate the striatum. METHODS. We studied a 59-year-old man with advanced Parkinson's disease who received bilateral grafts of fetal ventral mesencephalic tissue in the postcommissural putamen. The tissue came from seven embryos between 6 1/2 and 9 weeks after conception. The patient died 18 months later from a massive pulmonary embolism. The brain was studied with the use of tyrosine hydroxylase immunohistochemical methods. RESULTS. After transplantation, the patient had sustained improvement in motor function and a progressive increase in fluorodopa uptake in the putamen on PET scanning. On examination of the brain, each of the large grafts appeared to be viable. Each was integrated into the host striatum and contained dense clusters of dopaminergic neurons. Processes from these neurons had grown out of the grafts and provided extensive dopaminergic reinnervation to the striatum in a patch-matrix pattern. Ungrafted regions of the putamen showed sparse dopaminergic innervation. We could not identify any sprouting of host dopaminergic processes. CONCLUSIONS. Grafts of fetal mesencephalic tissue can survive for a long period in the human brain and restore dopaminergic innervation to the striatum in patients with Parkinson's disease. In the patient we studied, clinical improvement and enhanced fluorodopa with uptake on PET scanning were associated the survival of the grafts and dopaminergic reinnervation of the striatum.