During the fermentation of lactulose, short-chain fatty acids are formed with consequent lowering of the colon pH and modification of the microflora. Lactulose promotes the growth of lactic acid bacteria and bifidobacteria and, more specifically, Lactobacillus acidophilus in the colon. Lactulose and lactulose-containing products fermented with lactic acid bacteria lower colonic pH balancing intestinal microecology and normalizing intestinal transit. In animal studies, lactulose promotes a mainly Gram-positive faecal microflora, but large doses of lactulose may be associated with transient diarrhoea. Our studies indicate that lactulose with lactic acid bacteria effectively relieves constipation in human volunteers. Lactulose with lactic acid bacteria in a fermented diary product can balance and prevent radiotherapy-associated diarrhoea and intestinal side effects. Normalizing the intestinal flora and stabilizing mucosal integrity with lactulose has beneficial effects in intestinal disorders. Lactulose and lactic acid bacteria offer a promising ingredient combination for future functional and special dietary foods in treating intestinal disturbances.