This Phase 2 proof-of-concept study is a double-blind, randomized, placebo-controlled, 15-week investigation of ganaxolone versus placebo for the treatment of Posttraumatic Stress Disorder (PTSD). Up to 120 subjects will be enrolled and randomized to receive either ganaxolone or placebo for 6 weeks. After 6 wks of randomized treatment all subjects will continue for 6 wks on ganaxolone. The aim of the study is to assess the efficacy of ganaxolone compared to placebo for the treatment of PTSD symptoms after 6 weeks of treatment using the Clinician-Administered PTSD Rating Scale (CAPS). The second aim of the study is to evaluate the safety and tolerability of ganaxolone in the PTSD population.