Little research has been done on the validity of posttraumatic stress disorder (PTSD) diagnoses that are found in Department of Veterans Affairs (VA) administrative data, even though they are often used in VA research. We compared PTSD diagnoses found in VA administrative data with PTSD Checklist (PCL) scores self-reported by 4,777 newly diagnosed participants in a national postal survey study. Using PCL scores of at least 50 as the gold standard, we compared positive predictive values (PPVs) for at least one versus at least two PTSD diagnoses (found within 4 months of the first) in VA administrative data overall and by subgroups of interest: age, sex, and clinic where first diagnosed. The overall PPV was 75% for at least one PTSD diagnosis and 82% for at least two PTSD diagnoses. Similarly, the PPV significantly increased for all subgroup analyses when at least two PTSD diagnoses were used. The increase in PPV was greatest for those first diagnosed in primary care and for those older than 65. To select a sample of veterans with more definitive PTSD from administrative data, researchers should select those veterans with at least two PTSD diagnoses as opposed to at least one.