Attrition or dropout is the failure of a participant to complete, comply, or the prematurely discontinuation or discharge from treatment, resulting in lost data and affecting outcomes. This review of 10 years of adult posttraumatic stress disorder (PTSD) treatment outcome literature specific to Criterion A events of human origin examines how attrition is defined and addressed, methodologically and statistically. Of the 13 experimental or quasi-experimental studies, 11 report attrition information. Compared to treatment completers, attriters more often had elevated pretest scores on PTSD and other symptom measures. The characteristics of dropouts given the intentional nature of the traumatic events reviewed in this study are shared to inform clinical practice. Recommendations for consistent methods in examining, analyzing, and interpreting treatment outcome data are also discussed. (Published Abstract)