No current therapy for Parkinson disease has been shown to slow or reverse the progressive course of the disease. As a departure from traditional treatments, gene therapy approaches provide a new hope for realizing this long-sought goal; but before they can be widely employed for use in patients, they must first be submitted to the rigorous safety and efficacy standards of the clinical trial. Some of the challenges of gene therapy clinical trial design are similar to those in studies of conventional pharmacological agents and include addressing the heterogeneity of the disease, the need for clinical and surrogate endpoints, and the issue of distinguishing "symptomatic" from "neuroprotective" effects. Gene therapy trials also raise the issues of the risks of viral therapy, issues of dose-response, the need for sham surgery, and the long duration of risks and benefits. We conclude that the most feasible designs are for those treatments that are expected to produce a rapid improvement in directly observable symptoms. Trials of agents which are expected to produce only a slowing of progression and not a reversal of the disease course are likely to take much longer and will require the development of methods to assess quality of life and other non-motor aspects of the disease.