Fundamental pathological and neurochemical changes in Parkinson's disease are loss of midbrain dopamine neurons that innervate the caudate and putamen. In an effort to replenish the striatal dopaminergic innervation, fetal mesencephalic tissue containing dopamine cells was implanted into the unilateral putamen in two patients with severe Parkinson's disease. The tissue was obtained from three fetuses with gestational ages of 7 to 9 weeks. The cell suspension was stereotactically injected into the unilateral putamen using 5 needle trajectories. Postoperative immune suppression was not performed. Clinical improvement appeared after 2 months. Both patients showed improvement according to the Activities of Daily Living Scale during the off and practically-defined off state 9 and 14 months after surgery. The motor scores of the Unified Parkinson's Disease Rating Scale improved during the off and practically-defined off state 9 and 14 months after surgery. Dyskinesia and off state were shorter and less severe than before the transplantation. Although the long-term effects need to be ascertained, our short-term observation in these two patients with unilateral transplantation is encouraging and justifies further research trials in selected patients.