BACKGROUND & AIMS: Patients with irritable bowel syndrome (IBS) can be assigned to groups with different gastrointestinal (GI) symptoms based on results from a combined nutrient and lactulose challenge. We aimed to identify factors that predict outcomes to this challenge and to determine whether this can be used in noninvasive assessment of visceral sensitivity in patients with IBS. METHODS: We performed a prospective study of 100 patients with IBS diagnosed according to Rome III criteria (all subtypes) and seen at a secondary or tertiary care center. After an overnight fast, subjects were given a liquid breakfast (400 mL; Nutridrink) that contained 25 g lactulose. Before the challenge, we assessed visceral sensitivity (via rectal barostat), oro-anal transit time, and fecal microbiota composition (via 16S ribosomal RNA pyrosequencing); we determined IBS severity using questionnaires. The intensity of 8 GI symptoms, the level of digestive comfort, and the amount of exhaled H-2 and CH4 in breath were measured before and during a 4-hour period after the liquid breakfast. RESULTS: Based on the intensity of 8 GI symptoms and level of digestive comfort during the challenge, patients were assigned to groups with high-intensity GI symptoms (HGS; n = 39) or low-intensity GI symptoms (LGS; n [ 61); patients with HGS had more severe IBS (P <.0001), higher somatization (P <.01), and lower quality of life (P <.05-. 01) than patients with LGS. Patients with HGS also had significantly higher rectal sensitivity to random phasic distensions (P <.05-. 001, compared with patients with LGS). There were no significant differences between groups in fecal microbiota composition, exhaled gas in breath, or oro-anal transit time. CONCLUSIONS: We found, in a prospective study, that results from a lactulose challenge test could be used to determine visceral sensitivity and severity of IBS. The intensity of patient symptoms did not correlate with the composition of the fecal microbiota. The lactulose challenge test may help better characterize patients with IBS and evaluate the efficacy of new treatments. ClinicalTrial.gov no: NCT01252550.