Aliment Pharmacol Ther 2011; 33: 1059-1067 Summary Background There has been increasing interest in small intestinal bacterial overgrowth (SIBO) after reports of a link with irritable bowel syndrome (IBS), yet our understanding of this entity is limited. Aim Our aim was to estimate the yield of patients undergoing duodenal aspirate culture, and to identify symptoms and features that predict SIBO. Methods A medical chart review of patients who had undergone duodenal aspirate culture at an academic medical centre in 2003 was performed to record clinical characteristics and culture results. The associations between aspirate results and symptoms, medical diagnoses and medication use were assessed using logistic regression. Results A total of 675 patients had available aspirate results. Mean age of the sample was 53 (s.d. 17) and 443 (66%) were female patients. Overall, 8% of aspirates were positive for SIBO; 2% of IBS patients had SIBO. Older age, steatorrhoea and narcotic use were associated with SIBO (P < 0.05). PPI use was not associated with SIBO, but was associated with bacterial growth not meeting criteria for SIBO (P < 0.05). Inflammatory bowel disease (IBD), small bowel diverticula and pancreatitis were positively associated with an abnormal duodenal aspirate (P < 0.05), but other conditions including IBS were not associated with SIBO. Conclusion Older age, steatorrhoea, narcotic use, IBD, small bowel diverticula and pancreatitis were associated with small intestinal bacterial overgrowth based on abnormal duodenal aspirate culture results. However, no clear associations of true small intestinal bacterial overgrowth with IBS or PPI use were detected, in contrast to recent speculation.