Chronic abdominal pain (CAP) is an extremely pervasive childhood condition and, like IBS in adults, it is one of the functional bowel disorders without a clear framework of understanding or an effective treatment. However, new research suggests that small intestinal bacterial overgrowth (SIBO) may be the unifying pathophysiology that explains the variety of symptoms experienced by patients with IBS. As CAP in children is believed to be a precursor to IBS in adults, we hypothesize that children with this disorder have a significantly greater prevalence of small intestinal bacterial overgrowth (SIBO) than normal, healthy children, and that eradication of bacterial overgrowth with antibiotics will reduce symptoms of chronic abdominal pain in children with this condition. To prove this, we will first aim to determine the prevalence of SIBO in both healthy children and those with CAP. We will do this by performing a lactulose breath hydrogen test, the gold standard for the noninvasive measurement of SIBO, on 40 healthy controls and 80 subjects with CAP. We will then assess whether eradication of SIBO with antibiotics will reduce symptoms of chronic abdominal pain in children with this condition. To do this we will randomize, in a double-blinded fashion, the 80 CAP patients to receive a 10-day course of either the antibiotic Rifaximin or a placebo. After completion of the treatment we will evaluate all these patients for eradication of bacterial overgrowth by repeating a lactulose breath hydrogen test. We will also assess for symptom improvement by re-administering questionnaires.

Inclusion Criteria:

• Subjects must be between the ages of 8 and 18
• Subjects must be able to swallow pills
• Healthy Controls must be siblings, other family members or friends of the CAP subjects or children who are undergoing an overnight fast in order to obtain AM labs for another purpose other than this study.
• Female CAP subjects who are sexually active or who may become sexually active during the study will be required to practice an effective method of birth control (e.g., oral contraceptives, contraceptive patch or injection, IUD, double barrier method) before entering into the study.
• All CAP subjects must meet the Rome II Criteria for Functional Bowel Disorders Associated with Abdominal Pain or Discomfort in Children

Exclusion Criteria:

Subjects will be excluded if they:

• have a history of inflammatory bowel diseases, diabetes, cirrhosis or other liver disease, juvenile rheumatoid arthritis, systemic lupus, a history of bowel resection (including gastric, small bowel or colon; gallbladder surgery or appendectomy are NOT exclusion criteria).
• have been treated with antibiotics or probiotics within the past 2 months.
• have a history of TB infection or positive Mantoux test performed at screening
• have a history of allergy to rifampin or rifaximin

8 weeks