This study is being conducted to determine if losartan, an angiotensin receptor blocker (ARB), is safe and effective in the treatment of posttraumatic stress disorder (PTSD) symptoms. The study is also intended to determine if certain genetic markers are useful in predicting PTSD symptom reduction with losartan. Approximately 160 subjects with chronic PTSD ages 18-65 will participate in this study across five sites. Subjects will be assigned by chance to take either flexibly dosed losartan (up to a maximum dosage of 100 mg) or placebo (which resembles the study drug but has no active ingredients), once a day for 10 weeks. Furthermore, it is hypothesized that CC homozygotes for rs4311 SNP in the ACE gene will have a superior response to losartan on PTSD symptoms compared to T carriers.