AIM: To estimate the incidence of secondary lactase deficiency (SLD) in patients with postinfectious irritable bowel syndrome (PIBS) and the value of the small bowel microflora in its development and to elaborate treatment options for SLD/MATERIAL AND METHODS: One hundred and thirty-eight patients with PIBS, including 112 (81.2%) women and 26 (18.8%) men, were examined. The patients' mean age was 33.9±9.1 years. The duration of the disease was 2.6±1.4 years. Lactase deficiency (LD) was diagnosed using the color scale to test biopsy specimens from the duodenal retrobulbar region. The bacterial overgrowth syndrome (BOS) was identified by a 2-hour lactulose (20 ml) hydrogen breath test. Sixty patients with moderate SLD were randomized to 2 groups: 1) 41 patients received basic therapy (mesim forte as one tablet t.i.d., no-spa, 40 mg, t.i.d.) and combined probiotic bifiform (Ferrosan) containing Bifidobacterium longum 107, Enterococcus faecium 107 as one capsule t.i.d. for 14 days. Group 2 patients (n=19) had basic therapy in combination with placebo/RESULTS: SLD was detected in 59.4% of the patients with PIBS, including 43.5 and 15.9% with moderate and severe forms, respectively. In all cases, SLD was accompanied by BOS in the small bowel lumen, as confirmed by the results of a hydrogen breath test [101±37 ppm (a normal value of