Avoidance coping and symptoms of posttraumatic stress disorder (PTSD) covary. However, relatively little research has examined the bi-directional relation between these constructs among individuals in treatment for PTSD. The current longitudinal study examined the reciprocal associations between avoidance coping and PTSD symptom severity during and after residential PTSD treatment among a sample of 1073 military veterans (88.9% male; M_age = 52.39 years) with chronic, treatment-resistant PTSD. Greater avoidance coping at intake predicted more severe PTSD symptoms at discharge, and severity of PTSD symptoms at discharge predicted increased avoidance at follow-up. Conversely, PTSD symptom severity at intake was not related to avoidance coping at discharge, and in turn avoidance coping at discharge was not related to PTSD symptom severity at follow-up. These findings offer a number of important clinical implications including evidence suggesting avoidance may predict poorer treatment response among individuals seeking treatment for chronic PTSD, and that greater end-of-treatment PTSD symptom severity may predict increased avoidance following treatment.