BACKGROUND Small intestinal bacterial overgrowth (SIBO) may cause symptoms in patients with abdominal bloating, distension, and gas. SIBO can be assessed using the lactulose breath test (LBT). A commonly used probiotic supplement is Align containing Bifidobacterium infantis 35624. The aim of this study was to determine the effect of B. infantis 35624 on hydrogen and methane excretion during LBT. METHODS Healthy subjects underwent LBT before and after 2 weeks of daily Align administration. Hydrogen and methane concentrations were measured for each breath sample. Results are expressed as mean ± SE and analyzed using repeated measures ANCOVA. A breath test was considered positive if hydrogen and/or methane increased > 20 ppm above baseline by 90 min of the test or if a dual hydrogen peak was present. RESULTS Nineteen healthy subjects were studied. Hydrogen levels were similar pre- and post-probiotic across the 3-h study (p = 0.768). In contrast, methane levels were significantly higher with probiotic administration (p = 0.012). A rise in methane > 20 ppm was seen in three subjects pre-probiotic but six post-probiotic. Of the 19 subjects, an "abnormal" LBT pre-probiotic was present in ten subjects and during the probiotic, 13 were abnormal. CONCLUSIONS This study found that 2 weeks of B. infantis 35624 (Align) supplementation affects LBT assessment for SIBO by significantly increasing methane, but not hydrogen, excretion after lactulose administration. Methane levels reached values that would be considered positive for SIBO patients. This study suggests that patients undergoing LBT should discontinue probiotics prior to the test as these supplements may alter the test results.