Background Digital interventions offer a potentially cost-effective means to support patient self-management in primary care, but evidence for the feasibility, acceptability and cost-effectiveness of digital interventions remains mixed. This programme focused on the potential for self-management digital interventions to improve outcomes in two common, contrasting conditions (i.e. hypertension and asthma) for which care is currently suboptimal, leading to excess deaths, illness, disability and costs for the NHS. Objectives The overall purpose was to address the question of how digital interventions can best provide cost-effective support for patient self-management in primary care. Our aims were to develop and trial digital interventions to support patient self-management of hypertension and asthma. Through the process of planning, developing and evaluating these interventions, we also aimed to generate a better understanding of what features and methods for implementing digital interventions could make digital interventions acceptable, feasible, effective and cost-effective to integrate into primary care. Design For the hypertension strand, we carried out systematic reviews of quantitative and qualitative evidence, intervention planning, development and optimisation, and an unmasked randomised controlled trial comparing digital intervention with usual care, with a health economic analysis and nested process evaluation. For the asthma strand, we carried out a systematic review of quantitative evidence, intervention planning, development and optimisation, and a feasibility randomised controlled trial comparing digital intervention with usual care, with nested process evaluation. Setting General practices (hypertension, n = 76; asthma, n = 7) across Wessex and Thames Valley regions in Southern England. Participants For the hypertension strand, people with uncontrolled hypertension taking one, two or three antihypertensive medications. For the asthma strand, adults with asthma and impaired asthma-related quality of life. Interventions Our hypertension intervention (i.e. HOME BP) was a digital intervention that included motivational training for patients to self-monitor blood pressure, as well as health-care professionals to support self-management; a digital interface to send monthly readings to the health-care professional and to prompt planned medication changes when patients’ readings exceeded recommended targets for 2 consecutive months; and support for optional patient healthy behaviour change (e.g. healthy diet/weight loss, increased physical activity and reduced alcohol and salt consumption). The control group were provided with a Blood Pressure UK (London, UK) leaflet for hypertension and received routine hypertension care. Our asthma intervention (i.e. My Breathing Matters) was a digital intervention to improve the functional quality of life of primary care patients with asthma by supporting illness self-management. Motivational content intended to facilitate use of pharmacological self-management strategies (e.g. medication adherence and appropriate health-care service use) and non-pharmacological self-management strategies (e.g. breathing retraining, stress reduction and healthy behaviour change). The control group were given an Asthma UK (London, UK) information booklet on asthma self-management and received routine asthma care. Main outcome measures The primary outcome for the hypertension randomised controlled trial was difference between intervention and usual-care groups in mean systolic blood pressure (mmHg) at 12 months, adjusted for baseline blood pressure, blood pressure target (i.e. standard, diabetic or aged > 80 years), age and general practice. The primary outcome for the asthma feasibility study was the feasibility of the trial design, including recruitment, adherence, intervention engagement and retention at follow-up. Health-care utilisation data were collected via notes review. Review methods The quantitative reviews included a meta-analysis. The qualitative review comprised a meta-ethnography. Results A total of 622 hypertensive patients were recruited to the randomised controlled trial, and 552 (89%) were followed up at 12 months. Systolic blood pressure was significantly lower in the intervention group at 12 months, with a difference of –3.4 mmHg (95% confidence interval –6.1 to –0.8 mmHg), and this gave an incremental cost per unit of systolic blood pressure reduction of £11 (95% confidence interval £5 to £29). Owing to a cost difference of £402 and a quality-adjusted life-year (QALY) difference of 0.044, long-term modelling puts the incremental cost per QALY at just over £9000. The probability of being cost-effective was 66% at willingness to pay £20,000 per quality-adjusted life-year, and this was higher at higher thresholds. A total of 88 patients were recruited to the asthma feasibility trial (target n = 80; n = 44 in each arm). At 3-month follow-up, two patients withdrew and six patients did not complete outcome measures. At 12 months, two patients withdrew and four patients did not complete outcome measures. A total of 36 out of 44 patients in the intervention group engaged with My Breathing Matters [with a median of four (range 0–25) logins]. Limitations Although the interventions were designed to be as accessible as was feasible, most trial participants were white and participants of lower socioeconomic status were less likely to take part and complete follow-up measures. Challenges remain in terms of integrating digital interventions with clinical records. Conclusions A digital intervention using self-monitored blood pressure to inform medication titration led to significantly lower blood pressure in participants than usual care. The observed reduction in blood pressure would be expected to lead to a reduction of 10–15% in patients suffering a stroke. The feasibility trial of My Breathing Matters suggests that a fully powered randomised controlled trial of the intervention is warranted. The theory-, evidence- and person-based approaches to intervention development refined through this programme enabled us to identify and address important contextual barriers to and facilitators of engagement with the interventions. Future work This research justifies consideration of further implementation of the hypertension intervention, a fully powered randomised controlled trial of the asthma intervention and wide dissemination of our methods for intervention development. Our interventions can also be adapted for a range of other health conditions. Trial and study registration The trials are registered as ISRCTN13790648 (hypertension) and ISRCTN15698435 (asthma). The studies are registered as PROSPERO CRD42013004773 (hypertension review) and PROSPERO CRD42014013455 (asthma review). Funding This project was funded by the National Institute for Health and Care Research (NIHR) Programme Grants for Applied Research programme and will be published in full in Programme Grants for Applied Research; Vol. 10, No. 11. See the NIHR Journals Library website for further information.