The etiology of irritable bowel syndrome (IBS) remains elusive even though several genetic and environmental pathogenic factors have been reported. IBS is considered to be a functional disorder without any detectable lesions in the patient's bowel. However, many studies have demonstrated that a subset of IBS patients have low-grade inflammation and aberrant T-cell activation in their intestinal mucosa. To elucidate the immune mechanism underlying the mucosal inflammation in IBS, we focused on dental metal hypersensitivity, a T cell-mediated, delayed-type allergic reaction that causes oral contact mucositis and systemic cutaneous inflammation. We recruited 147 Japanese IBS patients and 22 healthy controls (HCs). The subjects underwent the in vitro lymphocyte stimulation test to quantify their sensitivity to zinc, gold, nickel, and palladium, the metals that have been commonly used in dentistry. A total of 56.5% of the IBS patients were hypersensitive to at least one metal species, whereas 31.8% of HC were hypersensitive to only a single metal species. The overall incidence of metal hypersensitivity was significantly higher for IBS patients than for HC. Furthermore, a significantly higher proportion of IBS patients were hypersensitive to zinc and/or nickel. The severity of the sensitivity to zinc and nickel was also significantly greater for IBS patients than for HC. There was no significant difference in the sensitization rates and the sensitivity among the IBS subtypes. This pilot study demonstrates that IBS patients have a significantly higher prevalence of hypersensitivity to zinc and nickel, suggesting the possible involvement of dental metal hypersensitivity in IBS pathogenesis in a subset of patients.