Background: Physical activity has been found to have positive effects on symptoms of posttraumatic stress disorder (PTSD). However, the importance and role of cardiorespiratory fitness (CRF) in relation to PTSD treatment outcome is not yet clear. Objective: The purpose of the present study was to test the hypothesis that CRF would increase following intensive trauma-focused treatment (TFT) of PTSD augmented with physical activity, and that improved CRF would be associated with a significant decline in PTSD symptoms. Method: One hundred-eight individuals with severe PTSD (72% women; mean age = 40.44, SD = 11.55) were enrolled in an intensive TFT programme of 8 days within 2 consecutive weeks that consisted of daily prolonged exposure (PE), eye movement desensitization and reprocessing (EMDR) therapy and 6 hours of physical activity each day. CRF levels were assessed at baseline and post-treatment with a 6-Minute Walk Test (6MWT) and, in a subsample, with a submaximal ergometer test (PWC 75% /kg). Severity of PTSD symptoms was measured with the PTSD Symptom Scale-Self Report (PSS-SR). Results: A significant increase in CRF from pre-to post-treatment and a significant decrease of PTSD-symptoms was found. However, CRF difference scores were not associated with treatment outcome. Conclusions: Although individuals with PTSD may show an increase in CRF following an intensive TFT programme augmented with physical activity and a decrease of PTSD-symptoms, the current findings do not support the notion that treatment outcome is related to CRF.