Relatively little research is available comparing the efficacy and tolerability of selective serotonin reuptake inhibitors (SSRIs) during continuation therapy. This investigation reports the differential effect of 6 months of treatment with sertraline versus paroxetine for symptoms of depression, quality of life, and personality outcomes. Outpatients with unipolar major depression (DSM-III-R) were randomly assigned to receive 24 weeks of double-blind treatment with flexible doses of paroxetine (20-40 mg) or sertraline (50-150 mg). Assessments included the Montgomery-Åsberg Depression Rating Scale (MADRS), the Clinical Global Impression Scale, the Battelle Quality of Life Questionnaire, and the Structured Clinical Interview for DSM-III-R Personality Disorders screen questionnaire. One hundred seventy-six patients (mean age, 43 years; 64% female; baseline MADRS, 30.3) were treated with sertraline and 177 patients (mean age, 42 years; 71% female; MADRS, 30.7) with paroxetine. Antidepressant efficacy during continuation therapy was sustained, with only 2% of patients receiving sertraline and 9% of patients receiving paroxetine suffering a relapse. Continuation therapy resulted in a substantial conversion of responders during short-term treatment to full remission: remitter rates increased from 52% to 80% for sertraline and from 57% to 74% for paroxetine. The improvements in quality of life were related to a reduced depression score. SSRI treatment had significant beneficial effects on both categorical and dimensional measures of personality. A logistic regression analysis identified early response (25% reduction in MADRS scores at week 2) as the most important predictor of treatment response, whereas high severity, chronicity, and poor baseline quality of life had no effect. Both treatments were well-tolerated, with sertraline having a somewhat lower side effect profile. Sertraline and paroxetine demonstrated comparable efficacy during short-term and continuation therapy. Treatment was associated with significant improvement in quality of life and with reductions in axis II personality psychopathology.