Autonomic failure is one of the criteria according to the second consensus statement for the diagnosis of multiple system atrophy (MSA). Gastrointestinal symptoms are frequent complaints in patients with MSA and may be associated with reduced gastrointestinal motility due to autonomic nervous system dysfunction. However, there are few reports on gastric emptying in patients with MSA. We investigated gastric emptying in 25 patients with MSA, 20 patients with sporadic adult-onset ataxia of unknown etiology (SAOA), and 20 healthy volunteers using the (13)C-acetate breath test. Gastric emptying function is estimated by this test as the half-emptying time (HET) and peak time of the (13)C-%-dose-excess curve (T (max)), with expirations collected for 4 h after a test meal and determination of (13)CO(2) content using an infrared (IR) spectrophotometer. The HET and T (max) of gastric emptying were significantly delayed in patients with MSA as compared to those in SAOA and controls (p < 0.01). The HET and T (max) were not significantly different between SAOA and controls. No correlation existed between the HET or T (max) and the duration or severity of the disease in MSA patients. These results suggested that gastric emptying was significantly delayed in patients with MSA, and the delay already appeared in the early stage of the disease. Delayed gastric emptying is one of the autonomic failures and may be a clinical marker of MSA.