Serial evaluation of regional cerebral blood flow (rCBF) was performed by brain single photon emission computed tomography (SPECT) 99Tcm-HMPAO (hexamethylpropylene amine oxime) in four patients with Parkinson's disease who showed motor complications associated with pre-operation L-dopa treatment and received stereotactic foetal tissue transplantation in the right caudate nucleus and putamen. The rCBF was studied 12 h off drug treatment before transplantation and 1, 6 and 12 months after operation. The study included four age matched healthy volunteers. In the pre-surgical studies, all cases showed hypoperfusion in the bilateral temporal and parietal regions. Also, in two patients moderate hypoperfusion was fond on the contralateral side most clinically affected. In the post-surgical studies, a generalized decrease of the cortical rCBF was assessed in all casesqhn studied one month after grafting, improving after six month. In the striatal region, no significant changes were observed in the study aafter one month, but there were differences after six months, when all patients showed a significant increase to the right side (per cent change of 16, 11, 9 and 14, respectively). In the study 12 months after grafting, no significant changes were found compared with the six month study. The changes in rCBF observed after surgery in all patients were accompanied by a progressive improvement in their neurological performance, mainly because the motor complications associated with L-dopa therapy and daily dose requirement were reduced, also improving the drug response. These results suggest that the increase of perfusion at the implantation site may be related to the graf's viability