Background Many adolescents with asthma have dysfunctional breathing and poor quality of life. Breathing retraining is recommended for symptom management and breathing efficiency. This trial evaluated the feasibility of conducting a definitive trial to evaluate the effectiveness and cost-effectiveness of a digital breathing retraining intervention for adolescents with asthma (Breathe4T – a mobile-friendly website). Specifically, recruitment, follow-up response rates, acceptability and uptake of the intervention and measures, as well as agreement between two quality of life questionnaires were measured. Methods Adolescents (12-17 years) with asthma and impaired quality of life were recruited via UK primary and secondary care clinics and randomised into two, unblinded groups. The intervention group accessed Breathe4T for 6 months whilst the control group gained access after 6 months. Measures included quality of life (paediatric asthma quality of life questionnaire and paediatric quality of life short form), asthma control (asthma control test), healthcare utilisation and demographics at baseline, 2 and 6 months. Website data and interviews explored experiences of the intervention. Results 64 adolescents were randomised. At 2 months 30.2% of participants returned data, however telephone calls improved the rate to 70.3% at 6-month follow-up Breathing retraining was acceptable to adolescents and was perceived to have various benefits. Conclusions The study demonstrates acceptability and feasibility of a future definitive trial to evaluate effectiveness and impacts of a breathing retraining website on quality of life. Implications for recruitment and maximising follow up rates were identified. These learnings are likely to be applicable to other adolescent studies.