INTRODUCTION Gastrointestinal symptoms may occur in 50% to 70% of patients with diabetes. OBJECTIVES The aim of this study was to evaluate the prevalence of small intestinal bacterial overgrowth (SIBO) in patients with diabetes, as well as the relationship between SIBO and metabolic control of diabetes and the presence of chronic complications of the disease. PATIENTS AND METHODS The study group included 148 patients with type 1 diabetes, treated in the years 2013-2015. The control group consisted of 41 healthy volunteers. The presence of SIBO was assessed with a noninvasive breath test using 20 g of lactulose suspended in 200 ml of water, with the assessment of exhaled hydrogen concentrations. The measurements were performed at 15-minute intervals in the first hour and at 30-minute intervals in the second hour of the test. A positive result was considered as the output value of exhaled hydrogen of 20 parts per million (ppm) or higher or an increase in the output value of the exhaled hydrogen of 12 ppm during the first 60 minutes of the test. RESULTS We observed a lower prevalence of SIBO in the study group in comparison with controls (56 patients [37.8%] vs 30 healthy volunteers [73%]; P = 0.006). In the logistic regression model, this association was independent of age, sex, body mass index, cigarette smoking, serum C-reactive protein concentrations, and estimated glomerular filtration rate (odds ratio, 0.26; 95% confidence interval, 0.10-0.68; P = 0.006). CONCLUSIONS The prevalence of SIBO in patients with type 1 diabetes is lower than that in healthy subjects. One of the possible causes might be the beneficial effect of nutritional therapy in patients with diabetes.