Eighteen of 19 patients who underwent autologous adrenal medullary transplantation to the right caudate nucleus have been followed up for 18 months. During the course of this study, a statistically significant improvement was noted in percent "on" time, percent "on" time without dyskinesia, activity of daily living (ADL) scores during the "on" stages, and ADL, motor, and Schwab-England scores during the "off" stages. Benefits tended to be maximal at 6 months and to gradually lessen thereafter, although statistically significant improvement in comparison with baseline was still present at 18 months for ADL, motor, and Hoehn-Yahr scores during the "off" stages. Almost all parameters had deteriorated by 18 months compared with 12 months, including those remaining significantly improved in comparison with baseline. These patterns were similar for each of the three participating centers. Complications were largely restricted to the perioperative period.