This research contributed to the development of a combined approach for exploring psychological and biological mechanisms underlying the functional gastrointestinal disorder, IBS. Effective laboratory techniques and biological sampling protocols were developed and validated ensuring the accurate measurement of biomarkers associated with IBS. These biological methods were incorporated into a psychological intervention for IBS. By monitoring the progress of patients in this program, this research provided further support for the efficacy of this psychological treatment in reducing the severity of IBS symptoms and initial insight into the possible mechanisms involved. This research has implications for improved treatment efficacy through the development of more targeted and effective integrated biological and psychological IBS treatment programs.