Background: Posttraumatic stress disorder (PTSD) is a significant problem for military veterans. There is an international imperative to improve access to effective treatments, but more research is needed to ascertain the extent to which treatments found to be efficacious in research settings translate to successful national implementation efforts. Method: This study reports the clinical outcomes for the first 100 clients treated following the implementation of cognitive processing therapy (CPT) across a national community-based veterans' mental health service that commenced in May 2012. The implementation included training and ongoing clinical supervision, leadership support, and updates to the service's data collection and intake system to support the delivery of CPT. The service implemented an intake screen (the Primary Care PTSD) that was used to allocate clients who screened positive for PTSD to CPT-trained therapists. An outcome measure for PTSD (the PTSD Checklist) was incorporated into the services' computerized records system. Clients who received CPT were assessed pretreatment and posttreatment. Results: Statistically significant and clinically large improvements were found for self-reported PTSD (effect size = 1.01, P <.001). In addition, the study obtained high levels of treatment fidelity in the delivery of the CPT treatment. Conclusions: There is relatively little published research supporting the effectiveness of evidence-based PTSD treatments following national implementation efforts. This is the first study to systematically report CPT treatment outcomes from a national implementation effort, using service-based outcome monitoring data. Results indicate that when administered as part of routine clinical practice, CPT achieves large clinically significant improvements for PTSD comparable with those found in randomized controlled trials.