Background: Human retinal pigment epithelial (RPE) cells produce levodopa and can be isolated from postmortem human eye tissue, grown in culture, and implanted into the brain attached to microcarriers. These implants ameliorated the motor deficits in rodent and nonhuman primate models of Parkinson disease. Objective: To evaluate the safety and efficacy of unilateral implantation of human RPE cells attached to gelatin microcarriers into the putamen contralateral to the more symptomatic side of patients with Parkinson disease. Design: Open-label pilot study. Setting: A tertiary referral center for movement disorders. Patients: Six patients with advanced Parkinson disease. Interventions: We performed stereotactic intrastriatal implantation of approximately 325 000 RPE cells on microcarriers. Main Outcome Measure: Change from baseline to 12 months in the Unified Parkinson's Disease Rating Scale motor subscore with the patients in the practically defined off state (not taking antiparkinsonian medications for at least 12 hours overnight). Results: The implants were well tolerated. We observed an average improvement of 48% at 12 months after implantation in the Unified Parkinson's Disease Rating Scale motor subscore with the patient in the off state, which was sustained through 24 months. Improvement was also observed in activities of daily living, quality of life, and motor fluctuations. No off-state dyskinesias were observed. Conclusions: Implants of human RPE cells attached to gelatin microcarriers appear to be safe and well tolerated, and they improved motor symptoms in patients with Parkinson disease. On the basis of these results, a randomized, double-blind, placebo-controlled study has been initiated. ©2005 American Medical Association. All rights reserved.