Background: Depression is a prevalent disorder, associated with a high disease burden and substantial societal,economic and personal costs. Cognitive behavioural treatment has been shown to provide adequate treatment fordepression. By offering this treatment in a blended format, in which online and face-to-face treatment are combined,it might be possible to reduce the number of costly face-to-face sessions required to deliver the treatment protocol.This could improve the cost-effectiveness of treatment, while maintaining clinical effects. This protocol describes thedesign of a pilot study for the evaluation of the feasibility, acceptability and cost-effectiveness of blended cognitivebehavioural therapy for patients with major depressive disorder in specialized outpatient mental health care.Methods/design: In a randomized controlled trial design, adult patients with major depressive disorder are allocated toeither blended cognitive behavioural treatment or traditional face-to-face cognitive behavioural treatment (treatment asusual). We aim to recruit one hundred and fifty patients. Blended treatment will consist of ten face-to-face and nine onlinesessions provided alternately on a weekly basis. Traditional cognitive behavioural treatment will consist of twentyweekly sessions. Costs and effects are measured at baseline and after 10, 20 and 30 weeks. Evaluations are directed atcost-effectiveness (with depression severity and diagnostic status as outcomes), and cost-utility (with costs per qualityadjusted life year, QALY, as outcome). Costs will encompass health care uptake costs and productivity losses due toabsence from work and lower levels of efficiency while at work. Other measures of interest are mastery, workingalliance, treatment preference at baseline, depressive cognitions, treatment satisfaction and system usability.Discussion: The results of this pilot study will provide an initial insight into the feasibility and acceptability of blendedcognitive behavioural treatment in terms of clinical and economic outcomes (proof of concept) in routine specializedmental health care settings, and an indication as to whether a well-powered clinical trial of blended cognitive behaviouraltreatment for depression in routine practice would be advisable. This will be determined based on the perspective ofvarious stakeholders including patients, mental health service providers and health insurers. Strengths and limitations ofthe study are discussed.Trial registration: Netherlands Trial Register NTR4650. Registered 18 June 2014.