OBJECTIVE: To perform an open trial on the effects of the extract of the dietary herb Rikkunshi-to (RKT) on gastroparesis in Parkinson's disease (PD) patients, using objective parameters given by the (13)C-sodium acetate expiration breath test (gastric emptying study). METHODS: Twenty patients with PD were enrolled into this study. Eleven patients were male and 9 were female, with the following characteristics (mean ± SD): age, 69.4 ± 8.17 years; disease duration, 4.34 ± 4.03 years; modified Hoehn and Yahr stage, 2.37 ± 0.98, and Unified Parkinson's Disease Rating Scale Part 3 motor score, 16.6 ± 7.37. Fourteen patients came to the clinic due to constipation; 16 patients were taking 288 ± 72 mg/day levodopa/carbidopa, 2 were taking dopamine agonists, and the others were not treated yet. All patients underwent the breath test. Statistical analysis was performed using Student's t test. RESULTS: RKT was well tolerated by all patients and none experienced abdominal pain or other adverse effects, except for its bitter taste. RKT significantly reduced the peak time of the (13)C-dose-excess curve (p < 0.05). CONCLUSION: In this pilot trial, we found a significant shortening of the gastric emptying time after administration of the dietary herb extract RKT in PD patients. Further studies examining both gastric emptying and delayed-on in PD are warranted. .