Senkyu-chacho-san (SC), an herbal medicine, can modulate striatal dopamine levels, but its effects on Parkinson's disease (PD) are unclear. To evaluate the efficacy, tolerance, and safety of SC in PD, 22 patients with idiopathic PD (aged 49 to 82 years; 13 women, staging Hoehen and Yahr 3 or 4) were given SC, 5.0 or 7.5g three times per day, for eight weeks. Existing antiparkinsonian drug regimes were kept unchanged. Motor functions were assessed based on the Unified Parkinson's Disease Rating Scale (UPDRS) at baseline and at weeks four and eight. After four weeks of SC administration, 14 patients showed a significant improvement in motor performance with respect to the motor scores of the UPDRS (p<0.05). The improvement in motor function declined slightly in four patients after eight weeks of treatment. No patients became worse in motor functions during treatment, and no serious adverse events occurred. This study suggests that SC is useful as an adjunct to dopaminergic drugs in PD.