Background Computerised cognitive-behavioural therapy (CCBT) might offer a solution to the current undertreatment of depression Aims To determine the clinical effectiveness of online, unsupported CCBT for depression in primary care. Method Three hundred and three people with depression were randomly allocated to one of three groups: Colour Your Life; treatment as usual (TAU) by a general practitioner; or Colour Your Life and TAU combined. Colour Your Life is an online, multimedia, interactive CCBT programme. No assistance was offered. we had a 6-month follow-up period. Results No significant differences in outcome between the three interventions were found in the intention-to-treat and per protocol analyses. Conclusions Online, unsupported CCBT did not outperform usual care, and the combination of both did not have additional effects. Decrease in depressive symptoms in people with moderate to severe depression was moderate in all three interventions. Online CCBT without support is not beneficial for all individuals with depression.