Autonomous neuropathy in patients with diabetes is associated with dysmotility and abdominal discomfort. The disturbances resemble to some extent those seen in patients with functional dyspepsia. To gain further insight into the disorders, we compared patients with long-standing diabetes, patients with functional dyspepsia, and healthy individuals with respect to abdominal symptoms, width of gastric antral area, and autonomic nerve function. We investigated 42 type I diabetic outpatients by structured interview for abdominal discomfort, ultrasonography of the gastric antrum, assessment of vagal and sympathetic nerve function by respiratory sinus arrhythmia and skin conductance, and measurement of blood sugar and HbA1c. Immediately after a standard meal of soup with meat, 21 (50%) of the 42 patients with diabetes complained of abdominal discomfort (pain, bloating, fullness), which was significantly less frequent (95% CI of difference 0.03-0.5) than previously seen in patients with functional dyspepsia (76%), and significantly more frequent (95% CI of difference 0.3-0.6) than that seen in healthy individuals (4%). Bloating was the most marked postprandial complaint. Mean fasting antral area was significantly wider in patients with diabetes (mean 4.9 cm2, SD 1.7) compared to healthy individuals (mean 3.5 cm2, SD 1.2), 95% CI of difference 0.6-2.2 cm2. Mean postprandial antral area was 14.8 cm2 (SD 4.6) in the patients with diabetes, which is insignificantly wider than in patients with functional dyspepsia (mean 13.0 cm2, SD 4.0) but significantly wider (95% CI of difference 1.9-6.5 cm2) than that seen in healthy individuals (mean 10.6 cm2, SD 3.8). The mean respiratory sinus arrhythmia was 0.7 beats/min (SD 0.7) in the patients with diabetes, which was insignificantly lower than that seen in patients with functional dyspepsia (2.1 beats/min, SD 4.5), and significantly lower (99% CI of difference 3.8-7.1 beats/min) compared to healthy individuals (6.2 beats/min, SD 3.8). It is concluded that patients with diabetes have a wider gastric antrum and more discomfort after a meal than healthy individuals. Compared to patients with functional dyspepsia, patients with diabetes have a wider postprandial antrum but fewer symptoms. The very low vagal tone seen in patients with diabetes may play an important role in the pathogenesis of their gastric motility disturbance and postprandial abdominal discomfort.