Synaptic dopamine release from embryonic nigral transplants has been monitored in the striatum of a patient with Parkinson's disease using [11 C]-raclopride positron emission tomography to measure dopamine D 2 receptor occupancy by the endogenous transmitter. In this patient, who had received a transplant in the right putamen 10 years earlier, grafts had restored both basal and drug-induced dopamine release to normal levels. This was associated with sustained, marked clinical benefit and normalized levels of dopamine storage in the grafted putamen. Despite an ongoing disease process, grafted neurons can thus continue for a decade to store and release dopamine and give rise to substantial symptomatic relief.