<qa>  What is the primary objective of the study?Irritable bowel syndrome IBS is a common gastrointestinal disorder affecting about 20 of the United States population and comprising almost 50 of referrals to gastroenterology practices. Although the pathophysiology of IBS is poorly understood, more recently, both inflammation and an increased intestinal permeability have been identified as potential factors in the etiology of diarrhea-predominant IBS patientsdIBS. Despite the potential of an inflammatory etiology for IBS, few studies have examined the efficacy of anti-inflammatory agents such as mesalamine in patients with IBS. The primary objective of our study is to determine the efficacy of AprisoTM Salix Pharmaceuticals Inc, a long-acting mesalamine, in the treatment of patients with dIBS. AprisoTM, is a mesalamine approved by the Food and Drug Administration FDA on October 31, 2008 for the maintenance of remission in patients with active, mild to moderate ulcerative colitis. The investigators will perform a randomized, double-blind, cross-over trial of mesalamine compared to placebo in patients with dIBS. This study will analyze both the objective and subjective measures of improvement in IBS symptoms. The subjective measures will include improvements in the overall symptom severity scores and the Global Improvement Scale GIS and the objective measures will include histological measures of inflammation as well as improvements in the intestinal membrane permeability of treated subjects. The investigators will demonstrate that during the 12 weeks treatment period with mesalamine, subjects will have improvements in their overall symptom scores, GIS scores, and will have a reduction in intestinal inflammation and also prohibit a likely improvement in Intestinal membrane permeability. 