BACKGROUND: Computerised cognitive behaviour therapy (cCBT) has been developed as an efficient form of therapy delivery with the potential to enhance access to psychological care. Independent research is needed which examines both the clinical effectiveness and cost-effectiveness of cCBT over the short and longer term. OBJECTIVES: To compare the clinical effectiveness and cost-effectiveness of cCBT as an adjunct to usual general practitioner (GP) care against usual GP care alone, for a free-to-use cCBT program (MoodGYM; National Institute for Mental Health Research, Australian National University, Canberra, Australia) and a commercial pay-to-use cCBT program (Beating the Blues(®); Ultrasis, London, UK) for adults with depression, and to determine the acceptability of cCBT and the experiences of users. DESIGN: A pragmatic, multicentre, three-armed, parallel, randomised controlled trial (RCT) with concurrent economic and qualitative evaluations. Simple randomisation was used. Participants and researchers were not blind to treatment allocation. SETTING: Primary care in England. PARTICIPANTS: Adults with depression who scored ≥ 10 on the Patient Health Questionnaire-9 (PHQ-9). INTERVENTIONS: Participants who were randomised to either of the two intervention groups received cCBT (Beating the Blues or MoodGYM) in addition to usual GP care. Participants who were randomised to the control group were offered usual GP care. MAIN OUTCOME MEASURES: The primary outcome was depression at 4 months (PHQ-9). Secondary outcomes were depression at 12 and 24 months; measures of mental health and health-related quality of life at 4, 12 and 24 months; treatment preference; and the acceptability of cCBT and experiences of users. RESULTS: Clinical effectiveness: 210 patients were randomised to Beating the Blues, 242 patients were randomised to MoodGYM and 239 patients were randomised to usual GP care (total 691). There was no difference in the primary outcome (depression measured at 4 months) either between Beating the Blues and usual GP care [odds ratio (OR) 1.19, 95% confidence interval (CI) 0.75 to 1.88] or between MoodGYM and usual GP care (OR 0.98, 95% CI 0.62 to 1.56). There was no overall difference across all time points for either intervention compared with usual GP care in a mixed model (Beating the Blues versus usual GP care, p = 0.96; and MoodGYM versus usual GP care, p = 0.11). However, a small but statistically significant difference between MoodGYM and usual GP care at 12 months was found (OR 0.56, 95% CI 0.34 to 0.93). Free-to-use cCBT (MoodGYM) was not inferior to pay-to-use cCBT (Beating the Blues) (OR 0.91, 90% CI 0.62 to 1.34; p = 0.69). There were no consistent benefits of either intervention when secondary outcomes were examined. There were no serious adverse events thought likely to be related to the trial intervention. Despite the provision of regular technical telephone support, there was low uptake of the cCBT programs. Cost-effectiveness: cost-effectiveness analyses suggest that neither Beating the Blues nor MoodGYM appeared cost-effective compared with usual GP care alone. Qualitative evaluation: participants were often demotivated to access the computer programs, by reason of depression. Some expressed the view that a greater level of therapeutic input would be needed to promote engagement. CONCLUSIONS: The benefits that have previously been observed in developer-led trials were not found in this large pragmatic RCT. The benefits of cCBT when added to routine primary care were minimal, and uptake of this mode of therapy was relatively low. There remains a clinical and economic need for effective low-intensity psychological treatments for depression with improved patient engagement. TRIAL REGISTRATION: This trial is registered as ISRCTN91947481. FUNDING: This project was funded by the National Institute for Health Research Health Technology Assessment programme.