Irritable bowel syndrome (IBS) and fibromyalgia (FM) are common functional diseases in adult women. The aim of this study was to investigate whether female predominance in IBS is related to FM. Fifty patients with IBS and 50 healthy controls were enrolled. All participants answered questionnaires including personal and medical history. In addition, psychiatric interviews were conducted. Patients were divided into two groups according to the coexistence of FM ( IBS+FM or IBS only). The data obtained from patients with or without FM and the control group were compared. There was a significant female predominance in patients with IBS+FM (83.4%, F:M=5:1), but IBS-only patients consisted mainly of males (59.4%, F:M=2:3) (P<0.01). Comparison of IBS+FM and IBS-only patients showed no significant difference in depression and anxiety status. However, both anxiety and depression scores were found to be higher in female IBS patients than their male counterparts (P<0.01 and P<0.05, respectively). Our findings suggest that the female predominance in IBS patients may result from coexisting FM.