Background: Thus far collaborative stepped care (CSC) studies have not incorporated self-help as a first step. Aims: To evaluate the effectiveness of CSC in the treatment of common mental disorders. Method: An 8-month cluster randomised controlled trial comparing CSC to care as usual (CAU) (Dutch Trial Register identifier NTR1224). The CSC consisted of a stepped care approach guided by a psychiatric nurse in primary care with the addition of antidepressants dependent on the severity of the disorder, followed by cognitive-behavioural therapy in mental healthcare. Results: Twenty general practitioners (GPs) and 8 psychiatric nurses were randomised to provide CSC or CAU. The GPs recruited 163 patients of whom 85% completed the post-test measurements. At 4-month mid-test CSC was superior to CAU: 74.7% (n = 68) v. 50.8% (n = 31) responders (P = 0.003). At 8-month post-test and 12-month follow-up no significant differences were found as the patients in the CAU group improved as well. Conclusions: Treatment within a CSC model resulted in an earlier treatment response compared with CAU.