Background: Dose-effect relationship data suggest that short-term psychotherapy is insufficient for many patients with chronic distress or personality disorders (complex mental disorders). Aims: To examine the comparative efficacy of long-term psychodynamic psychotherapy (LTPP) in complex mental disorders. Method: We conducted a meta-analysis of controlled trials of LTPP fulfilling the following inclusion criteria: therapy lasting for at least a year or 50 sessions; active comparison conditions; prospective design; reliable and valid outcome measures; treatments terminated. Ten studies with 971 patients were included. Results: Between-group effect sizes in favour of LTPP compared with less intensive (lower dose) forms of psychotherapy ranged between 0.44 and 0.68. Conclusions: Results suggest that LTPP is superior to less intensive forms of psychotherapy in complex mental disorders. Further research on long-term psychotherapy is needed, not only for psychodynamic psychotherapy, but also for other therapies.