Two patients with Parkinson's disease who underwent implantation of fetal mesencephalic tissue into the putamen were serially studied using positron emission tomography and {18F}6-L-fluorodopa ({18F}dopa). The uptake of {18F}dopa is related to the functional integrity of the presynaptic dopaminergic system. Preoperative studies revealed a marked decrease in putamen {18F}dopa uptake, with lesser involvement of the caudate. Two and 4 months, respectively, after operation, both patients demonstrated functional improvement, as described elsewhere. One patient was scanned 5, 8, and 13 months after the operation and the other was scanned 7 and 12 months after the operation. In both patients, {18F}dopa uptake increased within the operated putamen despite a progressive decrease in tracer uptake in the unoperated striatal structures. We believe that this increased uptake of {18F}dopa at the implantation site represents functional integrity within a surviving neural graft. While there has been little further clinical improvement beyond the fifth postoperative month, the uptake of {18F}dopa at the operation site in both patients has progressively increased. The kinetic data provide evidence of disease progression in the unoperated striatum, which balanced against increasing graft function, may explain why clinical improvement reached a plateau within months after surgery.