Background: Despite effective pharmacotherapy, asthma impairs the quality of life (QoL) of most patients. Non-pharmacologic approaches, including breathing retraining, are of great interest to patients but under-used due difficulties in accessing therapy Methods: A self-guided digital breathing retraining intervention delivered as a DVD plus printed booklet (DVDB) was developed and its effectiveness evaluated in a pragmatic 12-month, 3-armed RCT involving 655 adult asthmatics, compared to 3 ‘face-to-face’ respiratory physiotherapist sessions plus booklet (powered for equivalence), and with ‘usual care’ as control (powered for superiority). The primary outcome was Asthma Quality of Life Questionnaire (AQLQ) score, secondary outcomes included patient-reported and physiological measures of asthma, and healthcare costs. Results: AQLQ scores were significantly higher than control in the DVDB group; adjusted mean difference, 95% CI: 0.28 (0.11, 0.44), p Conclusions: Breathing retraining programmes improve QoL in patients with incompletely controlled asthma. They reduce healthcare costs, despite having no significant effect on lung function or airways inflammation. They can be delivered conveniently and cheaply as a self-guided digital audiovisual physiotherapy programme as add-on treatment.