During the pre-symptomatic stage of Parkinson's disease (PD), the idiopathic PD related abnormal synuclein immunostaining is confined to the medulla oblongata and olfactory bulb, according to Braak. In the study of the enteric nervous system of PD, it has reported that Lewy bodies were found in the Auerbach's and Meissner's plexuses. These lesions may cause dysfunction of the gastrointestinal tract (GI) as pre-clinical symptoms of PD. However, because L: -dopa therapy itself may worsen the symptoms of the digestive tract function, it is needed to evaluate the gastrointestinal tract function in patients with early-stage, untreated (de novo) PD. In the present study, using the (13)C-acetate breath test ((13)C-ABT), we investigated gastric emptying in 20 untreated, early-stage PD patients and 40 treated, advanced-stage PD patients, and 20 healthy volunteers. Gastric emptying was examined by the (13)C-ABT [the half emptying time (HET), the peak time of the (13)C% dose-excess curve (T (max))]. The T (max) and HET of gastric emptying as assessed using the (13)C-ABT was significantly delayed in untreated, early-stage PD patients as compared to the controls (P < 0.001). The T (max) and HET of gastric emptying were not significantly delayed in untreated, early-stage PD patients as compared to treated, advanced-stage PD patients. The results demonstrated that delay in gastric emptying did not differ between untreated, early-stage and treated, advanced-stage PD patients. Gastric emptying of untreated, early-stage PD is already delayed. Delayed gastric emptying may be one of markers of the pre-clinical stage of PD.