Fructo-oligosaccharides are naturally occurring sweet substances that are poorly absorbed and have the potential to be clinically useful nonnutritive sweeteners. Because most nonabsorbed carbohydrates are fermented yielding gas, we assessed flatulent symptoms and H2 excretion during ingestion of fructo-oligosaccharide (5 g tid) for 12 d. Ten subjects had significantly greater flatulence while taking the oligosaccharide than did five subjects taking sucrose (5 g tid). Breath H2 after 10 g fructo-oligosaccharide was similar to that of 10 g lactulose, suggesting near total malabsorption of the fructo-oligosaccharide. Although previous studies found a marked diminution in breath H2 after prolonged exposure to lactulose, breath H2 response increased by 50% after a 12-d period on the oligosaccharide and gaseous symptoms did not improve. We conclude that adaptation of colonic bacteria to carbohydrate malabsorption is variable and may depend upon quantity or nature of the carbohydrate.