BACKGROUND/AIMS: Lactulose breath test (LBT) has been used as a presumptive surrogate marker for small intestinal bacterial overgrowth (SIBO). However, recent reports suggest that abnormal LBT cannot discriminate patients with irritable bowel syndrome (IBS) from the control. Thus, the aim of this study was to evaluate the usefulness of LBT in IBS. METHODS: LBT from 76 IBS patients, 70 functional bowel disorders (FBD), and 40 controls were examined. LBT was considered positive if (1) baseline breath hydrogen (H2) >20 parts per million (ppm) or rise of breath H2 >20 ppm above the baseline in <90 mins, or (2) baseline breath methane (CH4) >10 ppm or rise of breath CH4 >10 ppm above the baseline in <90 mins. The subjects were categorized into predominant hydrogen producers (PHP), predominant methane producers (PMP), combined producer, and both negative group based on LBT. RESULTS: The rate of abnormal LBT in the IBS, FBD, and control group were 44.7%, 41.4%, and 40.0% respectively without significant differences. The rate of PHP or PMP was not significantly different among the IBS, FBD, and control group. When clinical characteristics were analyzed in IBS and FBD according to LBT types, IBS subtypes and symptoms were not significantly different. CONCLUSIONS: LBT was not useful to discriminate IBS/FBD patients from the control. The assessment of SIBO by LBT in IBS should be revalidated in the future.