Background & Aims: Abdominal bloating is a frequent complaint in irritable bowel syndrome (IBS), but its underlying mechanism remains uncertain. Our aim was to determine whether the abdominal wall, specifically its adaptation to intra-abdominal volumes, plays a role. Methods: In 12 patients complaining of abdominal bloating (8 IBS and 4 functional bloating) and in 12 healthy controls, the effect of colonic gas load (24 mL/min rectal gas infusion for 1 hour) on perception (measured by a 0-6 scale), abdominal girth, and muscular activity was tested. With the participants sitting on an ergonomic chair and the trunk erect, multichannel electromyography was measured via bipolar surface electrodes located over the upper and lower rectus abdominis, and the external and internal oblique bilaterally. Results: In healthy controls, colonic gas loads produced subjective symptoms (score, 3.0 ± 0.3), objective abdominal distention (girth increment, 6 ± 1 mm), and increased the activity of the abdominal muscles (external oblique activity, 11% ± 3% in; P < .05 vs basal). At the same infused gas volumes, the patients developed significantly more symptoms (score, 4.5 ± 0.4) and abdominal distention (11 ± 1 mm; P < .05 vs healthy for both). These abnormal responses were associated with failed tonic contraction of the abdominal wall (external oblique activity change, -1% ± 4%; P value not significant vs basal) and paradoxic relaxation of the internal oblique (activity reduction, 26% ± 7%; P < .01 vs basal). Conclusions: In patients with bloating, abdominal perception and distention in response to intra-abdominal volume increments are exaggerated markedly and associated with muscular dystony of the abdominal wall. © 2006 American Gastroenterological Association Institute.