Disclosed is a method of diagnosing irritable bowel syndrome, fibromyalgia, chronic fatigue syndrome, depression, attention deficit/hyperactivity disorder, autoimmune diseases, such as multiple sclerosis and systemic lupus erythematosus, or Crohn's disease, which involves detecting the presence of small intestinal bacterial overgrowth (SIBO) in a human subject having at least one symptom associated with a suspected diagnosis of any of those diagnostic categories. Also disclosed is a method of treating these disorders, and other disorders caused by SIBO, that involves at least partially eradicating a SIBO condition in the human subject. The method includes administration of anti-microbial or probiotic agents, or normalizing intestinal motility by employing a prokinetic agent. The method improves symptoms, including hyperalgesia related to SIBO and disorders caused by SIBO. Also disclosed is a kit for the diagnosis or treatment of irritable bowel syndrome, fibromyalgia, chronic fatigue syndrome, depression, attention deficit/hyperactivity disorder, autoimmune diseases, or Crohn's disease.