Although little bacteria colonize upper part of alimentary tract, its microflora is very diverse. In normal conditions intestinal bacteria remain in balance, they constitute barrier, protecting us from infections. Qualitative as well as quantitative disturbance of the microflora has a negative influence on digestion and absorption. Small intestine bacterial overgrowth (SIBO) is defined as an increased number of nonpathogenic bacteria – over 10⁵ organisms in 1 ml of small intestine content. The aim of the study is to assess the frequency of SIBA among children with abdominal symptoms. Material and methods: The study consisted of 83 children (43 girls and 40 boys) aged 4‑17 (mean 11±3.56 years). After excluding organic disorders, hydrogen breath test with lactulose (administered per os) was established among them. Expired air was analysed using Gastrolyzer by Bedfont. Results: Gastrointestinal symptoms were analysed. It appeared that abdominal pain was the most common symptom among 47 (54%) children (among them 27 girls and 20 boys). Eighteen (21%) patients presented additional symptoms that coexisted with abdominal pain. HBT result was positive among 59 (71%) patients – 32 (74%) girls and 27 (68%) boys. Conclusions: Among patients with abdominal pain about 70% presented positive result of HBT.