BACKGROUND Functional bowel disorder (FBD) may be caused by a decrease in disaccharidase activity. Thus, the timely diagnosis of disaccharidase deficiency could lead to a better prognosis in patients with this condition. AIM To determine the potential value of intestinal disaccharidases glucoamylase, maltase, sucrase, and lactase in understanding the etiology and pathogenesis of FBD. METHODS A total of 82 FBD patients were examined. According to the Rome IV criteria (2016), 23 patients had diarrhea-predominant irritable bowel syndrome (IBS), 33 had functional diarrhea, 10 had constipation-predominant IBS, 4 had functional constipation, and 12 had mixed IBS. The Dahlqvist method was used to measure disaccharidase activity in the brush-border membrane of mature enterocytes of the small intestine, in duodenal biopsies obtained during esophagogastroduodenoscopy. RESULTS Lactase deficiency was detected in 86.5% of patients, maltase deficiency in 48.7%, sucrase deficiency in 50%, and glucoamylase deficiency in 84.1%. The activities of all enzymes were reduced in 31.7% of patients, and carbohydrase deficiency was detected in 63.5% of patients. The low activity of enzymes involved in membrane digestion in the small intestine was found in 95.2% of patients. CONCLUSION In 78 of the 82 patients with FBD, gastrointestinal symptoms were associated with disaccharidase deficiency.