Dietary interventions in irritable bowel syndrome (IBS) include a traditional IBS diet following the guidelines from the National Institute for Health and Clinical Excellence and a diet low in fermentable oligo-, di-, monosaccharides and polyols (FODMAPs). The aim of this study was to evaluate the adherence to these diets, food groups difficult to replace, and dietary determinants of symptom improvement. Sixty-six patients with IBS were randomized to a 4-wk low FODMAP or traditional IBS diet. Participants completed 4-d diet diaries before and during the intervention and reported symptoms on the IBS severity scoring system. We described adherence to the diets on the food group and product level and investigated the association between adherence and symptom improvement. Adherence to the low FODMAP diet was good and consistent: All participants had a comparable shift in the diet's principal components compatible with the guidelines. Most high FODMAP products were well replaced with low FODMAP equivalents. However, total energy intake fell by 25%, mainly owing to a 69% decreased intake of snacks (P < 0.001). The traditional IBS diet did not shift the diet's principal components, and despite the guidelines, consumption of coffee and alcoholic beverages remained rather high (>50% of baseline). Total energy intake fell by 11% (P = 0.15). For both diets, there was a trend toward an association between adherence and symptom improvement (P < 0.10). In both the low FODMAP and traditional IBS diet, certain food groups were difficult to replace. Because adherence may predict symptom improvement, close dietary guidance might enhance the efficacy of both diets.