We studied 14 patients with irritable bowel syndrome for the presence of increased intestinal permeability to food antigens and their responses to diet with and without disodium cromoglycate. After a standardized oral challenge with cow milk, serum beta-lactoglobulin was increased above control values in three patients. This finding did not correlate with response to hypoallergenic diet or treatment with disodium cromoglycate for 3 weeks. However over 50% of patients improved after diet with and without DSCG (2/5 on diet only and 5/7 with disodium cromoglycate of 12 evaluable cases). Since only two patients had elevated serum IgE levels, our results suggest that intolerance rather than hypersensitivity to foods may play a role in the disease. The tests we used to identify immunologic mechanisms could not predict which patients would do better on the diet and/or the drug.