Several recent reports have suggested that foetal ventral mesencephalic transplants could alleviate motor symptoms in patients with Parkinson's disease. Expectations of future success must be clarified by precise analysis of the extent and limitation of recovery associated with an assessment of function of the graft using [18F]fluorodopa (18F-dopa) PET. Two patients with idiopathic Parkinson's disease, severely impaired despite optimal medication, have been followed 10 and 17 months after stereotaxic unilateral intrastriatal transplantation of neural cells dissociated from human foetal ventral mesencephalon. Analysis of the clinical evolution complied with the protocol established in the 'Core Assessment Program for Intracerebral Transplantation'. Both patients have benefited from the transplantation in their daily activities and in motor timed tests, although they still exhibit parkinsonian symptoms and require L-dopa therapy. This is associated with a gradual increase in 18F-dopa uptake at the site of grafting. There are two major clinical changes: (i) a bilateral motor improvement for the speed of movements (the quality of movements improved almost exclusively on the side contralateral to the graft); (ii) a change in the outcome of the L-dopa treatment as exemplified by a postoperative transient period of heavy dyskinesias and subsequent additive actions of the two treatments. These results confirm that neural transplantation may be useful for patients with Parkinson's disease. The improvement recorded on the side ipsilateral to the graft does not match that observed on the contralateral side and it is proposed that bilateral transplantation may be necessary. The existence of a transient postoperative period of heavy dyskinesias suggests that the patients may benefit from a controlled decrease of L-dopa intake after grafting.