Investigated the contribution of personality traits to length of treatment in a behavior therapy-oriented outpatient treatment setting. All 328 patients (aged 18-77 yrs) with anxiety and/or mood disorders admitted during a 1-yr period filled out a personality questionnaire before the start of treatment, and were followed for at least 1 yr. It was found that Introverts stayed much longer in therapy and got more treatment sessions than Extraverts. Level of Neuroticism did not influence length of treatment. Low Extraversion scores may be a reflection of more severe and enduring (trait-like) psychiatric symptomatology. Also, Introverts may need more time to engage in a therapeutic relationship. ((c) 1997 APA/PsycINFO, all rights reserved)