Different groups worldwide have observed in recent years that stereotactic implantation of fetal tissue can ameliorate the clinical symptoms of Parkinson's disease. The authors therefore investigated whether implantation of fetal ventral mesencephalic (FVM) tissue via open surgery is also capable of producing an improvement and whether this improvement is transient or long lasting. The authors report their findings in a 5-year follow-up study in 10 patients with Hoehn and Yahr Grade IV or V Parkinson's disease in whom a single FVM graft was implanted in a cavity created in the right caudate nucleus. The results indicate that the implants improved motor function and that clinical recovery persisted in seven of the 10 patients 5 years after implantation. Amelioration was observed in both the on and off phases and was accompanied by a 64% reduction in the levodopa dose and withdrawal of the dopamine agonist. The on phase was prolonged from 39% of the waking day to 72%, with reduced intensity and duration of dyskinesias. All symptoms that were analyzed showed improvement, although they differed in intensity and time of onset. The course of improvement seemed to be stepwise, with significant improvement between 5 and 7 months postimplantation followed by two waves of progress peaking in Months 15 and 36. Withdrawal of cyclosporine in three patients after more than 2 years of administration produced a decline in the patients' clinical conditions. In conclusion, the results indicate that open surgery implantation of FVM tissue in the caudate nucleus improves the clinical condition of parkinsonian patients and that this improvement can persist for at least 5 years. In comparison with two earlier series reported by the authors, which involved implants of perfused adrenal medulla and coimplantation of adrenal medulla and peripheral nerve, the course and pattern of improvement in these implant recipients suggests that their recovery can be attributed to more than one factor.