Food hypersensitivity as a cause of irritable bowel syndrome was investigated by means of exclusion diet and blind provocation. Twelve atopic and twelve non-atopic individuals entered into the study. Skin prick testing with 20 foods and food additives and RAST specific for food only, were done in all cases. Serum IgE level was also measured. In 14 patients one or several food or additives were shown to induce the typical symptoms of IBS. In at least nine cases of atopy, an IgE-mediated mechanism could be incriminated. Among other potential pathogenetic mechanisms, the presence in the intestinal tract of yeast (Candida albicans, Geotrichum candidum) seems to be of major importance. Yeast apparently favor the development of allergic as well as pseudo-allergic reactions, at least in some patients. Finally, at least in atopic patients complaining of IBS, it is of importance to search for a food component. Dramatic clinical improvements can result from the introduction of an adequate exclusion diet.