OBJECTIVE: The use of e-health interventions has grown in demand due to their accessibility, low implementation costs and their potential to improve the health and well-being of people across a large geographical area. Despite these potential benefits, little is known about the cost-effectiveness of self-guided e-health interventions. The aim of the study was to compare the cost and consequences of 'iSupport', an e-health intervention to reduce mental health issues in dementia carers. DESIGN: A cost-consequence analysis (CCA) of a multi-centre, single-blind randomised controlled trial of iSupport. The CCA was conducted from a public sector (National Health Service, social care and local authority) perspective plus a wider societal perspective. Delivery costs of iSupport were collected using a bottom-up micro-costing approach. SETTING: 352 participants were recruited from three centres in England, Wales and Scotland. PARTICIPANTS: Participants eligible for inclusion were adults over the age of 18 years who self-identified as an unpaid carer with at least 6 months of experience caring for an individual with a diagnosis of dementia. Between 12 November 2021 and 31 March 2023, 2332 carers were invited to take part in the study. 352 participants were randomised: 175 randomised to the iSupport intervention group and 177 to the usual care control group. The mean age of participants in the intervention and control groups was 63 and 62, respectively. MAIN OUTCOME MEASURES: The CCA presented the disaggregated costs and health-related quality of life measured using the EuroQol five-dimension. RESULTS: There was no significant difference in generic health-related quality of life measured using the EQ-5D-5L (p=0.67). Both groups reported higher mean costs between baseline and 6 months, but the change in costs was significantly lower in the intervention group. Between baseline and 6 months, the mean change in total resource use costs from the public sector perspective was significantly different between groups (p=0.003, r=-0.161) reporting a mean change per participant of £146 (95% CI: -33 to 342) between the intervention and control groups. From the wider societal perspective, there was no significant difference (p=0.23) in the mean change in total resource use and informal care costs between the two groups from baseline to 6 months. CONCLUSION: Use of iSupport was associated with reduced health and social care resource use costs for carers compared with care-as-usual. Self-guided e-health interventions for dementia carers may have the potential to reduce health and social care resource use and wider societal costs, but evidence relating to their effectiveness and cost-effectiveness is lacking. TRIAL REGISTRATION NUMBER: ISRCTN17420703.