Participants with IBS are skin patch tested (no needles) to 80 different foods and food additives, compounded for patch testing, in search of food allergies. The testing requires 3 office visits within a 4 or 5 day period. The patches are taped to the back at Visit #. At Visit #2 48 hours later, the patches are removed from the skin and the outside border of each patch is marked with a felt tip marker. At Visit #3 (final visit) 1 or 2 days later, the patch test reading is performed by the doctor-investigator. An allergy is identified as a small red mark where the food was in contact with the skin for 48 hours. Those participants found to have food allergies are then placed on an avoidance diet (no calorie restriction) for 16 weeks where they either avoid eating the food(s) to which they are allergic (the \"true\" avoidance diet) or food(s) to which the testing did not show an allergy (this is called the \"sham\" avoidance diet). There is a 50/50 chance of going on either avoidance diet. The avoidance diet is assigned in such a way that neither the participant or the doctor-investigator knows which diet is being followed. After the 16 weeks, the participants answers a brief online questionnaire that asks about the IBS symptoms while following the avoidance diet. After the 16 week avoidance diet and final questionnaire are completed, those participants who were on the sham diet will be told of their true food allergies which they may try avoiding on their own.