BackgroundAlthough self-help, internet-based cognitive behaviour therapy (ICBT) can produce significant reductions in anxiety for many young people, a sizable proportion show low program adherence and continue to show anxiety problems after treatment. It is important that we identify factors that predict those most likely to complete program sessions and benefit from self-help ICBT so that we can better match interventions to individual needs, and inform program design and development.ObjectiveThis study examined the role of social support in predicting treatment adherence and outcome among youth enrolled in an open-access, internet-delivered, CBT intervention targeting anxiety.MethodsParticipants were 3684 young people (aged 7 to 17 years) who reported elevated levels of anxiety symptoms when registering for the program. Treatment adherence was assessed as number of treatment sessions completed. Treatment response was evaluated as change in self-reported anxiety symptoms. Youth-reported social support (from friends, family and a special person) at pre-treatment was examined as a hypothesized predictor of adherence and outcome after controlling for baseline anxiety, gender, and age, with age being examined as a moderator.ResultsLinear regression analyses showed that participants with greater social support, from all sources, tended to show greater program adherence, although the proportion of explained variance was small. Age moderated the effect of family support upon adherence, such that greater family support was associated with greater number of sessions completed only for older youth. Greater family and total support were associated with greater reductions in anxiety, irrespective of participant age, but again the percent of variance explained was small. Younger participants were more likely to complete more sessions and to show greater reductions in anxiety. Those with higher pre-treatment anxiety tended to complete fewer sessions but demonstrated greater reductions in anxiety.ConclusionsThe findings highlight the need to consider ways to enhance treatment adherence and outcome of those engaging in self-help ICBT for youth anxiety problems, particularly when social support is low.