To assess the efficacy of fetal mesencephalic grafts in Parkinson's disease, it is important to know if the grafted cells survive and are functional. Positron emission tomography (PET) and 18Ffluorodopa (18Fdopa) have been used to demonstrate the survival of the grafted cells, but the relationship of 18Fdopa uptake changes in the grafted striatum to motor function remains unclear. We investigated this question with 16 serial PET scans in 5 severe parkinsonian patients unilaterally grafted in whom we found a significant and progressive increase of the 18Fdopa uptake in the grafted putamen. The number of patients was too small to assess the sensitivity of 18Fdopa PET scans in individual patients. Yet, by analyzing the 16 serial PET scans we found a correlation between the 18Fdopa uptake (Ki) in the grafted putamen and the percentage of daily time spent "on," suggesting that Ki changes have a functional meaning. In addition, the Ki values were correlated with the contralateral finger dexterity to the same extent in both the grafted and nongrafted putamen. These results indicate that 18Fdopa uptake reflects the motor function of the opposite side of the body, similarly in the grafted and ungrafted putamen, at least in terms of these tasks. Finally, extrapolating from these correlations offers the suggestion that clinical optimal results of the graft could be achieved if the graft brings the Ki values in the putamen to about two standard deviations of mean control values.