The present study was designed to determine whether augmenting sertraline with prolonged exposure (PE) would result in greater improvement than continuation with sertraline alone. Outpatient men and women with chronic PTSD completed 10 weeks of open label sertraline and then were randomly assigned to five additional weeks of sertraline alone (n = 31) or sertraline plus 10 sessions of twice-weekly PE (n = 34). Results indicated that sertraline led to a significant reduction in PTSD severity after 10 weeks but was associated with no further reductions after five more weeks. Participants who received PE showed further reduction in PTSD severity. This augmentation effect was observed only for participants who showed a partial response to medication.