Background: There is some evidence to suggest that computerised Cognitive Behavioural Therapy (cCBT) is effective for reducing adolescent anxiety and depression. However, most studies have used samples of young people in the general population or those who are 'at risk' of anxiety or depression, and so less is known about the usefulness of cCBT for adolescents with clinically significant levels of distress and impairment. Aims: This study aimed to investigate the feasibility of using a novel computerised cCBT intervention, 'Pesky gNATs,' with adolescents aged between 13 and 18 years old who had clinically significant symptoms of anxiety and/or depression. Method: Eleven participants were recruited from a Tier 3 Child and Adolescent Mental Health service (CAMHS). Six participants were allocated to the intervention group and received the intervention immediately, and five were allocated to the control group and received the intervention after a delay. Outcome measures were completed pre- and post- intervention. Qualitative data were also obtained during the final session. Results: A high participation and retention rate was found for the intervention and the majority of participants described the programme as helpful and identified several benefits of completing the programme. No differences in overall self-reported symptoms of anxiety and depression, self-concept and functioning were found when comparing the intervention and control group on pre- and post- intervention scores. However, decreases in parent-reported symptoms of anxiety and depression and the impact of symptoms on functioning were found across both groups. Conclusion: This study demonstrates the acceptability of using a cCBT programme in a Tier 3 CAMHS setting, but shows limited evidence that this intervention is effective at reducing self-reported symptoms of anxiety and depression. Therefore, 'Pesky gNATs' may not be appropriate as a standalone treatment for young people with more severe clinical presentations.