Aims: 1) To assess the IgG4 antibody responses to specific food antigens of patients with inflammatory bowel disease (ffiD) when compared to healthy controls. 2) To investigate the clinical and symptomatic impact of a 4-week IgG4-guided exclusion diet in IBD patients. Methods: Ninety-two patients with IBD were recruited. They were assessed for disease severity using validated questionnaires and inflammatory markers were tested. Their serum was tested for IgG4 reactions to specific food antigens and compared to healthy controls. Results: Sixty-six patients completed IgG4 guided exclusion diets. The Crohn's patients showed higher IgG4 antibody titres to egg white, egg yolk and potato when compared to healthy controls (only potato after using Bonferrino adjustment). The ulcerative colitis group showed higher IgG4 antibody titres to pork antigens and a lower response to cheese than the control group (though none were significant after Bonferrino adjustment). Tailored diets excluding the foods with the highest IgG4 antibody titres led to symptomatic improvement in both the Crohn's and ulcerative colitis groups. This was more marked in the Crohn's group. In the moderately active Crohn's group 8 of 14 achieved remission. There was a significant drop in ESR, post-diet in the Crohn's group. There was significant improvement in all components of the mCDAI (except presence of a mass). In patients with moderately active ulcerative colitis, an improvement was achieved with 4 of 6 patients achieving remission. There was a significant reduction in the frequency of bowel opening in the UC group and the ileoanal pouch group. Conclusions: Differences exist in IgG4 responses to specific food antigens between those with ulcerative colitis and Crohn's disease and healthy controls. The clinical relevance of these variations, remain of doubtful significance. Diets based around individuals IgG4 responses to specific foods have significantly improved both the disease activity index and the ESR in Crohn's patients. These diets reduce Seo scores and the CRP in patients with ulcerative colitis and benefit those with moderately active disease. Whilst it is difficult to pinpoint specific food intolerances, with advances such as IgG4 testing we hope to prolong remission in these patients and potentially reduce the need for pharmacotherapy and surgery. It not known if the mechanism of action is through alteration of intestinal microbiota or by reducing the effect of food antigenicity.