BackgroundUp to 10% of children and young people have a specific phobia that can significantly affect their mental health, development and daily functioning. Cognitive–behavioural therapy-based interventions remain the dominant treatment, but limitations to their provision warrant investigation into low-intensity alternatives. One-session treatment is one such alternative that shares cognitive–behavioural therapy principles but has a shorter treatment period.ObjectiveThis research investigated the non-inferiority of one-session treatment to cognitive–behavioural therapy for treating specific phobias in children and young people. The acceptability and cost-effectiveness of one-session treatment were examined.DesignA pragmatic, multicentre, non-inferiority randomised controlled trial, with embedded economic and qualitative evaluations.SettingsThere were 26 sites, including 12 NHS trusts.ParticipantsParticipants were aged 7–16 years and had a specific phobia defined in accordance with established international clinical criteria.InterventionsParticipants were randomised 1 : 1 to receive one-session treatment or usual-care cognitive–behavioural therapy, and were stratified according to age and phobia severity. Outcome assessors remained blind to treatment allocation.Main outcome measuresThe primary outcome measure was the Behavioural Avoidance Task at 6 months’ follow-up. Secondary outcomes included the Anxiety Disorder Interview Schedule, Child Anxiety Impact Scale, Revised Children’s Anxiety and Depression Scale, a goal-based outcome measure, Child Health Utility 9D, EuroQol-5 Dimensions Youth version and resource usage. Treatment fidelity was assessed using the Cognitive Behaviour Therapy Scale for Children and Young People and the One-Session Treatment Rating Scale.ResultsA total of 274 participants were recruited, with 268 participants randomised to one-session treatment (n = 134) or cognitive–behavioural therapy (n = 134). A total of 197 participants contributed some data, with 149 participants in the intention-to-treat analysis and 113 in the per-protocol analysis. Mean Behavioural Avoidance Task scores at 6 months were similar across treatment groups when both intention-to-treat and per-protocol analyses were applied [cognitive–behavioural therapy: 7.1 (intention to treat), 7.4 (per protocol); one-session treatment: 7.4 (intention to treat), 7.6 (per protocol); on the standardised scale adjusted mean difference for cognitive–behavioural therapy compared with one-session treatment –0.123, 95% confidence interval –0.449 to 0.202 (intention to treat), mean difference –0.204, 95% confidence interval –0.579 to 0.171 (per protocol)]. These findings were wholly below the standardised non-inferiority limit of 0.4, which suggests that one-session treatment is non-inferior to cognitive–behavioural therapy. No between-group differences in secondary outcome measures were found. The health economics evaluation suggested that, compared with cognitive–behavioural therapy, one-session treatment marginally decreased the mean service use costs and maintained similar mean quality-adjusted life-year improvement. Nested qualitative evaluation found one-session treatment to be considered acceptable by those who received it, their parents/guardians and clinicians. No adverse events occurred as a result of phobia treatment.LimitationsThe COVID-19 pandemic meant that 48 children and young people could not complete the primary outcome measure. Service waiting times resulted in some participants not starting therapy before follow-up.ConclusionsOne-session treatment for specific phobia in UK-based child mental health treatment centres is as clinically effective as multisession cognitive–behavioural therapy and highly likely to be cost-saving. Future work could involve improving the implementation of one-session treatment through training and commissioning of improved care pathways.Trial registrationThis trial is registered as ISRCTN19883421.FundingThis project was funded by the National Institute for Health and Care Research (NIHR) Health Technology Assessment programme and will be published in full inHealth Technology Assessment; Vol. 26, No. 42. See the NIHR Journals Library website for further project information.