Background: Little is known about the cost-effectiveness of preventing mental disorders. Aims: To study the cost-effectiveness of care as usual plus minimal contact psychotherapy relative to usual care alone in preventing depressive disorder. Method: An economic evaluation was conducted alongside a randomised clinical trial. Primary care patients with sub-threshold depression were assigned to minimal contact psychotherapy plus usual care (n=107) or to usual care alone (n=109). Results: Primary care patients with sub-threshold depression benefited from minimal contact psychotherapy as it reduced the risk of developing a full-blown depressive disorder from 18% to 12%. In addition, this intervention had a 70% probability of being more cost-effective than usual care alone. A sensitivity analysis indicated the robustness of these results. Conclusions: Over I year adjunctive minimal contact psychotherapy improved outcomes and generated lower costs. This intervention is therefore superior to usual care alone in terms of cost-effectiveness.