UNLABELLED: The purpose of this study is to evaluate the efficacy of a diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs) in adolescents with irritable bowel syndrome (IBS) and functional abdominal pain-not otherwise specified (FAP-NOS) in a non-guided setting, resembling clinical practice. This prospective multicenter cohort study conducted in 13 centers included patients aged 12-18 years diagnosed with IBS or FAP-NOS. Patients received educational material on FODMAPs, including extensive lists of high and low FODMAP foods and additional online information. They were instructed to replace high FODMAP foods with low FODMAP alternatives for the duration of 4 weeks. No dietician was consulted. The primary end point was the proportion of patients with treatment success (≥ 30% reduction of abdominal pain intensity) at 4 weeks. The key secondary outcome was adequate relief of IBS/FAP-NOS symptoms. Of the 325 included patients, 81 patients (24.9%) achieved treatment success (≥ 30% reduction of abdominal pain intensity) after 4 weeks, with higher rates in patients with IBS (29.3%) than FAP-NOS (16.8%, OR 2.16 (1.04-4.48)). Adequate relief was reported in 51 patients (15.7%). There was a significant decrease in abdominal pain intensity (2.2 (1.1) vs. 2.5 (1.0), P < 0.001), daily bloating (2.4 (2.1) vs. 2.8 (2.3), P < 0.001), and flatulence (2.4 (2.1) vs. 2.8 (2.3), P = 0.001). Adverse events were mild and infrequent. CONCLUSION: The low FODMAP diet in a non-guided setting, mimicking clinical practice, yielded treatment success in almost 30% adolescents with IBS and 17% in FAP-NOS, suggesting it may not be the first treatment option for these patients. TRIAL REGISTRATION: EUCTR2015-003293-32-NL. WHAT IS KNOWN: • Irritable bowel syndrome (IBS) and functional abdominal pain-not otherwise specified (FAP-NOS) are common disorders in children which negatively impact quality of life. • While a diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs) has demonstrated effectiveness in adult IBS, its efficacy in pediatric IBS and FAP-NOS remains uncertain. • Clinical application of the low FODMAP diet often occurs without dietician consultation, contrary to controlled trial settings. WHAT IS NEW: • The low FODMAP diet, without dietician guidance, resulted in treatment success in almost 30% of adolescents with IBS and only 17% with FAP-NOS. • With only 15.7% of participants achieving adequate relief of IBS/FAP-NOS symptoms, the non-guided low FODMAP diet may not be the first treatment option for pediatric IBS and FAP-NOS.