We aimed to discover whether psychological treatment for generalized anxiety disorder (GAD) was associated with changes in the big five personality traits and their facets.

Patients with GAD were randomized either to receive cognitive behaviour therapy (CBT, n = 28) or metacognitive therapy (MCT, n = 32). Before and after 12 sessions of treatment, 55 of the patients completed the full Revised NEO Personality Inventory (NEO‐PI‐R) (240 items).

Patients with GAD showed a personality profile with high Neuroticism and lower Extraversion and Openness. Treatment across conditions was associated with significant reduction in Neuroticism and increased Extraversion and Openness. There were no significant changes in Agreeableness and Conscientiousness. However, their facets of Actions and Trust increased. Post‐treatment levels of neuroticism were associated with symptoms of worry before and after therapy, whereas post‐treatment extraversion was related to depressive symptoms after treatment. MCT was associated with greater reduction of Neuroticism than CBT.

This is the first study to show that efficient treatment for a specific disorder resulted in changes across NEO‐PI‐R factors and facets and that more efficient treatment results in greater change. If this reflects a reduced trait vulnerability for mental disorder, this might provide evidence of relapse prevention.