AIM To elucidate the effects of small intestinal bacterial overgrowth (SIBO) on the severity and complications of acute pancreatitis (AP). METHODS In total, 208 patients with AP as defined by the revised Atlanta classification were admitted to Xuanwu Hospital of Capital Medical University from 2013 to 2016. All patients were admitted within 72 h of AP onset. The hydrogen breath test was performed 7 d after AP onset to detect hydrogen production and evaluate the development of SIBO. The incidence of SIBO was analyzed in patients with AP of three different severity grades. The association between SIBO and complications of AP was also assessed. RESULTS Of the 27 patients with severe AP (SAP), seven (25.92%) developed SIBO. Of the 86 patients with moderately severe AP (MSAP), 22 (25.58%) developed SIBO. Of the 95 patients with mild AP (MAP), eight (8.42%) developed SIBO. There were significant differences in the rates of SIBO among patients with AP of different severities. Additionally, more severe AP was associated with higher rates of SIBO positivity (P < 0.05). SIBO in patients with AP mainly occurred within 72 h of the onset of AP. The incidence of organ failure was significantly higher in patients with SIBO than in those without (P < 0.05). CONCLUSION SIBO occurs more frequently in patients with MSAP or SAP than in those with MAP, usually <= 72 h after AP onset. Additionally, SIBO is associated with organ failure.