Background Although non-operative treatment is known to be effective for the treatment of uncomplicated acute appendicitis in children, randomised trial data comparing important outcomes of non-operative treatment with those of appendicectomy are lacking. Objectives The objectives were to ascertain the feasibility of conducting a multicentre randomised controlled trial comparing the clinical effectiveness and cost-effectiveness of a non-operative treatment pathway with appendicectomy for the treatment of uncomplicated acute appendicitis in children. Design This was a mixed-methods study, which included a feasibility randomised controlled trial, embedded and parallel qualitative and survey studies, a parallel health economic feasibility study and the development of a core outcome set. Setting This study was set in three specialist NHS paediatric surgical units in England. Participants Children (aged 4–15 years) clinically diagnosed with uncomplicated acute appendicitis participated in the feasibility randomised controlled trial. Children, their families, recruiting clinicians and other health-care professionals involved in caring for children with appendicitis took part in the qualitative study. UK specialist paediatric surgeons took part in the survey. Specialist paediatric surgeons, adult general surgeons who treat children, and children and young people who previously had appendicitis, along with their families, took part in the development of the core outcome set. Interventions Participants in the feasibility randomised controlled trial were randomised to a non-operative treatment pathway (broad-spectrum antibiotics and active observation) or appendicectomy. Main outcome measures The primary outcome measure was the proportion of eligible patients recruited to the feasibility trial. Data sources Data were sourced from NHS case notes, questionnaire responses, transcribed audio-recordings of recruitment discussions and qualitative interviews. Results Overall, 50% (95% confidence interval 40% to 59%) of 115 eligible patients approached about the trial agreed to participate and were randomised. There was high acceptance of randomisation and good adherence to trial procedures and follow-up (follow-up rates of 89%, 85% and 85% at 6 weeks, 3 months and 6 months, respectively). More participants had perforated appendicitis than had been anticipated. Qualitative work enabled us to communicate about the trial effectively with patients and families, to design and deliver bespoke training to optimise recruitment and to understand how to optimise the design and delivery of a future trial. The health economic study indicated that the main cost drivers are the ward stay cost and the cost of the operation; it has also informed quality-of-life assessment methods for future work. A core outcome set for the treatment of uncomplicated acute appendicitis in children and young people was developed, containing 14 outcomes. There is adequate surgeon interest to justify proceeding to an effectiveness trial, with 51% of those surveyed expressing a willingness to recruit with an unchanged trial protocol. Limitations Because the feasibility randomised controlled trial was performed in only three centres, successful recruitment across a larger number of sites cannot be guaranteed. However, the qualitative work has informed a bespoke training package to facilitate this. Although survey results suggest adequate clinician interest to make a larger trial possible, actual participation may differ, and equipoise may have changed over time. Conclusions A future effectiveness trial is feasible, following limited additional preparation, to establish appropriate outcome measures and case identification. It is recommended to include a limited package of qualitative work to optimise recruitment, in particular at new centres. Future work Prior to proceeding to an effectiveness trial, there is a need to develop a robust method for distinguishing children with uncomplicated acute appendicitis from those with more advanced appendicitis, and to reach agreement on a primary outcome measure and effect size that is acceptable to all stakeholder groups involved. Trial registration Current Controlled Trials ISRCTN15830435. Funding This project was funded by the National Institute for Health Research (NIHR) Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 25, No. 10. See the NIHR Journals Library website for further project information.