Small intestinal bacterial overgrowth syndrome is defined as an increased number of non-pathogenic bacteria over 10⁵ microorganisms in one millilitre of intestinal contents in the initial part of the small intestine. Predisposing disorders include e.g. drug-induced hypochlorydia, congenital and acquired defects of the gastrointestinal tract, immunodeficiency, severe stress situations as well as intestinal microflora imbalance after immunosuppressive therapy. The aim of the study was to assess the incidence of small intestinal bacterial overgrowth in children receiving cyclosporine A due to idiopathic nephritic syndrome. Material and methods: The study included 20 children (11 girls and 9 boys) aged 4–16 years (mean age 8.83 ± 3.75 years), diagnosed with idiopathic nephritic syndrome and treated with cyclosporine for over 3 months. The use of antibiotics or probiotics less than 3 months prior to the study was an exclusion criterion. Serum levels of cyclosporin A were measured in all patients. Hydrogen breath test with lactulose was performed as an additional examination. The exhaled breath was analysed using Gastrolyzer (Bedfont). A minimum increase of 20 hydrogen molecules per million air molecules between the maximum value and the fasting level or values in the first hour of the test was considered as a positive test outcome, indicating small intestinal bacterial overgrowth. Results: Negative result of hydrogen breath test, excluding small intestinal bacterial overgrowth, was observed in all 20 children. Conclusions: The administration of second line immunosuppressive agents in children with idiopathic nephritic syndrome does not induce non-pathogenic bacterial multiplication in the small intestine.