Two studies suggest that reductions in posttraumatic symptoms (Aderka et al., 2013) and cognitions (Zalta et al., 2014) precede reductions in depressive symptoms during prolonged exposure (PE) therapy for posttraumatic stress disorder (PTSD) in female assault survivors. The present study explored the temporal relationship between posttraumatic and depressive symptoms in a randomized trial of D-Cycloserine (DCS) versus placebo augmented virtual reality exposure (VRE) therapy for chronic World Trade Center-related PTSD following the September 11, 2001 terrorist attacks. Twenty-five male and female participants were randomly assigned to receive either 100 mg DCS (N = 13) or placebo (N = 12) 90 min before 12 weekly VRE sessions. Participants contributed a total of 280 weekly PTSD Checklist (PCL; Weathers et al., 1993) and Beck Depression Inventory-second edition (BDI-II; Beck et al., 1996) symptom scores. Two sets of mediation analyses for longitudinal mixed models assessed the effects of 1) lagged PCL on BDI-II (Model 1), and 2) lagged BDI-II on PCL (Model 2) in the VRE-DCS and VRE-Placebo treatment groups, respectively. Results revealed reciprocal relations between posttraumatic and depressive symptoms during VRE treatment, although reductions in posttraumatic symptoms led to subsequent reductions in depressive symptoms to a greater extent than the converse. These effects were stronger in the DCS-enhanced group. Findings suggest that VRE primarily decreases posttraumatic symptoms, which in turn leads to decreased depressive symptoms, and that DCS may strengthen these effects.