This study has some limitations. First, given the behavior-based treatments, participants were aware of their intervention assignments. This awareness may have introduced biases in the results, since persons interested in participating may have had positive expectations about the benefits of exercise. Second, we did not include a nonexercise control group, so the net gain of tai chi training cannot be gauged. However, the results of this trial show that tai chi is more effective than low-intensity, lowimpact exercise programs in alleviating the symptoms of Parkinson’s disease and improving functional ability. Finally, all participants were tested during “on” periods, which may have masked underlying changes induced by the training interventions.