IMPORTANCEAlthough it is widely believed that concealment or deception is required to elicit a placebo response, recent studies with adults suggest that open-label placebo (OLP) (ie, honestly prescribed placebos) can yield significant benefits. No studies of OLP have been performed with children.OBJECTIVE To evaluate the efficacy of OLP for the treatment of children and adolescents with functional abdominal pain or irritable bowel syndrome.DESIGN, SETTING, AND PARTICIPANTS This multicenter crossover randomized clinical trial was conducted from July 1, 2015, to June 15, 2018, at 3 US centers among children and adolescents aged 8 to 18 years with functional abdominal pain or irritable bowel syndrome defined per Rome III criteria. Statistical analysis was performed from March 1, 2019, to September 30, 2020, on an intention-to-treat basis.INTERVENTIONS Patients completed 1 week of observation prior to randomization to 1 of 2 counterbalanced groups: OLP for 3 weeks followed by a 3-week control period or control period for 3 weeks followed by OLP for 3 weeks. During the OLP period, participants took 1.5 mL of an inert liquid placebo twice a day. A standardized method for explaining the OLP was used, and the interaction with clinicians had the same duration and style for both time periods. Hyoscyamine was allowed as a rescue medication. MAIN OUTCOMES AND MEASURESThe primary outcome was the mean daily pain score during each of the interventions, measured on a 0-to 100-mm visual analog scale, where higher scores indicated greater pain. The number of rescue medications taken during each intervention served as an objective secondary measure.RESULTS Thirty patients (mean [SD] age, 14.1 [3.4] years; 24 female participants [80.0%]; 16 [53.3%] with functional abdominal pain and 14 [46.7%] with irritable bowel syndrome) completed the study. The mean (SD) pain scores were significantly lower during OLP treatment compared with the control period (39.9 [18.9] vs 45.0 [14.7]; difference, 5.2; 95% CI, 0.2-10.1; P = .03). Patients took nearly twice as many hyoscyamine pills during the control period compared with during the OLP period (mean [SD] number, 3.8 [5.1] pills vs 2.0 [3.0] pills; difference, 1.8 pills; 95% CI, 0.5-3.1 pills). CONCLUSIONS AND RELEVANCEDuring OLP, patients with functional abdominal pain or irritable bowel syndrome reported significantly less pain and took significantly fewer pain medications. Open-label placebo may be an effective treatment for children and adolescents with functional abdominal pain or irritable bowel syndrome.