We studied differences in diagnostic stability between patients with full and patients with partial posttraumatic stress disorder (PTSD). We collected self-reported symptoms of PTSD, anxiety, depression, and functioning at a Veterans Affairs mental health clinic (n = 1962). We classified patients as meeting full or partial PTSD based upon their initial assessment. We performed Kaplan-Meier survival analysis to compare stability of diagnosis over time and Cox proportional hazards models to understand how comorbid symptoms and level of functioning confounded the relationship. We performed a chart review to examine differences in treatment received by the two groups. Patients in the partial PTSD group lost their diagnosis significantly faster and at significantly higher rates than did patients with full PTSD. Comorbid symptoms contributed significantly to this difference. Mental health treatments delivered to the two groups were similar. These diagnoses appear to be different, suggesting that people with partial PTSD may benefit from a different clinical approach.