Objectives Exposure‐based psychotherapies for posttraumatic stress disorder (PTSD) are effective for many, but not all patients. It is important to determine for whom these treatments work and to examine predictors of success. Method An 8‐week modified prolonged exposure (PE) treatment, including components of behavioral activation and reducing the number of imaginal exposure sessions, was administered to a sample of 231 Veterans (mean age = 45.7 years, standard deviation = 14.89). Growth mixture modeling was used to model PTSD symptom trajectories across the 8‐week intervention and a postintervention appointment. Further, baseline demographics, social support, clinician‐rated PTSD symptoms, anxiety, and depression were examined as predictors of trajectories. Results Three classes emerged, labeled responders (n = 35), nonresponders (n = 190), and immediate responders (n = 6). The only significant baseline difference between responders and nonresponders was higher anxiety symptoms in the nonresponders. At follow‐up time points, there were higher levels of clinician‐rated PTSD, anxiety, and depression symptoms and lower social support in the nonresponders compared to the responders. Conclusion Findings suggest that modifying standard PE treatments by reducing imaginal exposure sessions while adding behavioral activation may not be advisable for most Veterans with PTSD.