The study examined the efficacy of sertraline, compared with placebo, in sustaining improvement and preventing relapse over 28 weeks in patients with posttraumatic stress disorder (PTSD) who had completed a 12-week double-blind, placebo-controlled acute treatment study and a subsequent 24-week open-label study of continuation treatment with sertraline. Ninety-six patients were randomly assigned, in a double-blind design, to 28 weeks of maintenance treatment with sertraline (50–200 mg, N=46; 78% were women) or placebo (N=50; 62% were women). Measures used in biweekly assessments included the Clinician-Administered PTSD Scale, the Impact of Event Scale, and the Clinical Global Impression severity and improvement ratings. Kaplan-Meier analyses were used to estimate time to discontinuation from the study due to relapse, relapse or study discontinuation due to clinical deterioration, and acute exacerbation. Continued treatment with sertraline yielded lower PTSD relapse rates than placebo (5% versus 26%). Patients who received placebo were 6.4 times as likely to experience relapse as were patients who received sertraline. Kaplan-Meier analyses confirmed the protective effect of sertraline in significantly extending time in remission. The ability of sertraline to sustain improvement was comparable across the three core PTSD symptom clusters (reexperiencing/intrusion, avoidance/numbing, and hyperarousal). A regression analysis found early response during acute treatment to be associated with a more than 16-fold reduced risk of relapse after placebo substitution. Sertraline, at a mean endpoint dose of 137 mg, was well tolerated, with no sertraline-related adverse events observed at a rate of 10% or higher. The results provide evidence for the ability of sertraline both to sustain improvement in PTSD symptoms and to provide prophylactic protection against relapse.