Inclusion criteria: IBS patients 18-80 years old, diagnosis of IBS; exclusion: Severe cardiac, pulmonary, liver or kidney disease, bloody stool After fasting for 12 hours, all subjects ingest 10 grams of lactulose. Hydrogen production, measured as hydrogen concentration in breath, is estimated at 10-minute intervals for a period of 240 minutes. The orocecal transit transit time is measured from ingestion of the lactulose solution until hydrogen peaks to 10 and 20 ppm when passing the ileocecal valve, being fermented by the cecal microflora. Data are compared to those of healthy volunteers.