The relationship between abdominal pain and bowel gas from bacterial fermentation of undigested carbohydrate was investigated in nine patients with irritable bowel syndrome (IBS), six lactose malabsorbers, and 11 asymptomatic controls. All subjects took breath samples and marked analog scales for abdominal pain, bloating, and psychological stress hourly during all waking hours for 7 days. Breath samples were analyzed for hydrogen concentration within 3 days, and the concentration was corrected for storage time. Symptoms of pain and bloating were significantly more common in IBS patients than in lactose malabsorbers or normal controls, and pain was significantly correlated with bloating in IBS patients. Breath hydrogen concentration was similar in all three groups, and breath hydrogen was not correlated with pain ratings in IBS patients. Thus, abdominal pain may be related to bloating from gastrointestinal gas, but bacterial fermentation cannot be the cause of such gas. The most likely source is swallowed air. This study also demonstrates the feasibility of monitoring hydrogen production in the bowel in field studies by having subjects collect hourly breath samples.