We evaluated 32 patients with Parkinson's disease in a double-blind, parallel group, placebo-controlled study with ciladopa (a troponylpiperazine derivative). The dosages administered were 5 mg b.i.d. and 15 mg b.i.d. Significant improvement was found in the gait scores and in the total disability scores in the high dose group (p<0.05). In addition, there was a trend toward improvement, though not significant, in the bradykinesia and rigidity scores in the high dose group and in the disability scores in the low dose group. Four patients in the low dose group and five patients in the high dose group decreased their Sinemet doses while no patients increased their Sinemet dose. There were no adverse effects observed in this study. These findings suggest that ciladopa may be an efficacious agent in Parkinson's disease. The low incidence of adverse effects with this agent suggests that higher doses may be utilized and may prove to be more effective.