The large intestine contains five types of endocrine cells that regulate its functions by sensing its luminal contents and releasing specific hormones. Chromogranin A (CgA) is a common marker for the gastrointestinal endocrine cells, and it is abnormal in irritable bowel syndrome (IBS) patients. Most IBS patients relate their symptoms to certain food elements. The present study investigated the effect of dietary guidance on the total endocrine cells of the large intestine as detected by CgA in 13 IBS patients. Thirteen control subjects were also included. Each patient received three sessions of dietary guidance. Colonoscopies were performed on controls and patients (at baseline and at 3-9 months after receiving guidance). Biopsy samples from the colon and rectum were immunostained for CgA and quantified by computerized image analysis. The densities of CgA cells in the total colon (mean ± SEM) among the controls and the IBS patients before and after receiving dietary guidance were 83.3 ± 10.1, 38.6 ± 3.7, and 64.7 ± 4.2 cells/mm(2), respectively (P = 0.0004), and were unchanged in the rectum. In conclusion, the increase in CgA cell density after receiving dietary guidance may reflect a change in the densities of the large intestinal endocrine cells causing an improvement in the IBS symptoms.