Despite empirical evidence demonstrating the effectiveness of collaborative stepped care program (SCP) in Western countries, such programs have not been evaluated in the east, which has a different services system structure and cultural nuances in seeking help for mental illness. Furthermore, only a few studies have used SCP for depression and anxiety prevention. We conducted a trial to test its effectiveness in preventing major depressive disorder and generalized anxiety disorder among primary care patients with subthreshold depression and/or anxiety in Hong Kong.