Specific foods were found to provoke symptoms of irritable bowel syndrome (IBS) in 14 of 21 patients. In 6 patients who were challenged double blind the food intolerance was confirmed. No difference was detected in changes in plasma glucose, histamine, immune complexes, haematocrit, eosinophil count, or breath hydrogen excretion produced after challenge or control foods. Rectal prostaglandin E2 (PGE2), however, increased significantly, and in a further 5 patients rectal PGE2 correlated with wet faecal weight. Food intolerance associated with prostaglandin production is an important factor in the pathogenesis of IBS.