There is a need for studies on colonic fermentation in order to learn more abouthealth and diseases of the colon. The aim of the present study was to evaluate the fate of twodifferent doses of fructo-oligosaccharides (5 and 15 g/d) v. glucose in the intestine of healthy men. Twenty-four volunteers participated in a 5-weekstudy. The study was a completely balanced multiple crossover trial using an orthogonal Latin-square design for three periods, with supplement periods of 7 d and two 7 d wash-out periods. Breath samples and faecal samples were collected. There was a clear gaseous response to the consumption of fructo-oligosaccharides. The highest dose significantly increased 24 h integratedexcretion of breath H2 (P < 0·05). Breath H2 excretion after ingestion of 5 g fructo-oligosaccharides was higher than control, but did not reach significance. No effects on the total concentration of short-chain fatty acids in faeces were observed, no modification of the molar proportions of the various short-chain fatty acids was observed. The faecal pH did not change. No changes in faecal weight were observed. No fructo-oligosaccharides were recovered in faeces. We conclude that fructo-oligosaccharides added to the diet of young Western subjects are fully metabolized in the large intestine. The level of fermentation seems to be dose-dependent.