Aim. To establish the rate of lactase deficiency (LD) in patients with post-infectious irritable bowel syndrome (PI-IBS), to define a role of enteric bacteria in the pathogenesis of hypolactasia, and to evaluate the efficiency of probiotic therapy. Subjects and methods. Examinations were made in 386 patients with PI-IBS, including 112 (79.4%) women; mean age 33.9±9.1 years; disease duration 2.6±1.4 years. Rapid tests of small intestinal mucosa (SIM) biopsy specimens obtained from the duodenal retrobulbar segment were used to diagnose LD. Bacterial growth was estimated by a hydrogen breath test using a Н2 MICRO gas analyzer. Results. The patients with PI-IBS were revealed to have moderate and severe LD in 25.6 and 10.9%, respectively. All the patients with LD were detected to have small intestinal (SI) bacterial overgrowth (BOG). An inverse correlation was found between LD and the degree of SI BOG (r=–0.53; p<0.001). 73.7% of the patients with moderate LD showed a positive effect of probiotic therapy as regression of clinical symptoms of LD, a decrease of hydrogen levels in expired air from 72.4±25.1 to 16.4±13.2 ppm (р<0.05), an increase of lactate activity in the SIM biopsy specimens and an improvement of quality of life from 2.69±0.53 to 5.53±0.64 scores according to the GCI scale. No improvement occurred in 73.8% of the patients with severe LD. Conclusion. LD was identified in 36.5% of the patients with PI-IBS. There was an inverse correlation between the degree of LD and SI BOG. The good therapeutic effect of probiotics in LD suggests that the symbiotic gut microflora positively affects the activity of lactase in the human SIM. No therapeutic effect of probiotics in patients with severe LN serves as the basis for a search for more active probiotic therapy.