ObjectivesInternet-delivered exposure-based cognitive behaviour therapy (ICBT) has been shown to be effective in the treatment of severe health anxiety. The health economic effects of the treatment have, however, been insufficiently studied and no prior study has investigated the effect of ICBT compared with an active psychological treatment. The aim of the present study was to investigate the cost effectiveness of ICBT compared with internet-delivered behavioural stress management (IBSM) for adults with severe health anxiety defined as Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) hypochondriasis. ICBT was hypothesised to be the more cost-effective treatment.SettingThis was a cost-effectiveness study within the context of a randomised controlled trial conducted in a primary care/university setting. Participants from all of Sweden could apply to participate.ParticipantsSelf-referred adults (N=158) with a principal diagnosis of DSM-IV hypochondriasis, of whom 151 (96%) provided baseline and post-treatment data.InterventionsICBT or IBSM for 12 weeks.Primary and secondary measuresThe primary outcome was the Health Anxiety Inventory. The secondary outcome was the EQ-5D. Other secondary measures were used in the main outcome study but were not relevant for the present health economic analysis.ResultsBoth treatments led to significant reductions in gross total costs, costs of healthcare visits, direct non-medical costs and costs of domestic work cutback (p=0.000–0.035). The incremental cost-effectiveness ratio (ICER) indicated that the cost of one additional case of clinically significant improvement in ICBT compared with IBSM was $2214. The cost-utility ICER, that is, the cost of one additional quality-adjusted life year, was estimated to be $10 000.ConclusionsICBT is a cost-effective treatment compared with IBSM and treatment costs are offset by societal net cost reductions in a short time. A cost-benefit analysis speaks for ICBT to play an important role in increasing access to effective treatment for severe health anxiety.Trial registration numberNCT01673035; Results.