Background & Aims: Patients reporting abdominal bloating exhibit impaired tolerance to intestinal gas loads. The aim of this study was to identify the gut compartment responsible for gas retention. Methods: In 30 patients predominantly reporting abdominal bloating (24 with irritable bowel syndrome and 6 with functional bloating) and 22 healthy subjects, gas (nitrogen, carbon dioxide, and oxygen) was infused into the intestine for 2 hours while measuring rectal gas outflow. First, in 12 patients and 10 healthy subjects, gas transit (24 mL/min jejunal infusion labeled with 74 MBq bolus of 133Xe) was measured by scintigraphy. Second, in groups of patients and healthy subjects, the effects of gas infusion (12 mL/min) in the jejunum versus ileum, jejunum versus cecum, and jejunum versus sham infusion (n = 6 each) were compared by paired tests. Results: In patients, total gut transit of gas was delayed (50% clearance time, 33 ± 4 min vs 23 ± 4 min in healthy subjects; P < .05) owing to impaired small bowel transit (50% clearance time, 20 ± 2 min vs 12 ± 3 min in healthy subjects; P < .05), whereas colonic transit was normal (50% clearance time, 13 ± 2 min vs 11 ± 2 min in healthy subjects; not significant). Furthermore, jejunal gas infusion in patients was associated with gas retention (329 ± 81 mL vs 88 ± 79 mL in healthy subjects; P <. 05), whereas direct ileal or colonic infusion was not (61 ± 103 mL and -143 ± 87 mL retention, respectively). Conclusions: In patients reporting bloating, the small bowel is the gut region responsible for ineffective gas propulsion. © 2005 by the American Gastroenterological Association.