The goal of this clinical trial is to compare a novel, massed treatment format of prolonged exposure to gold standard trauma focused cognitive behavioral therapy (weekly delivered prolonged exposure) for post-traumatic stress disorder. The objective with this study is to investigate efficacy, mechanisms of change and cost effectiveness of intensive prolonged exposure (I-PE) in regular Swedish psychiatric care. Adult patients with post-traumatic stress disorder will be randomly assigned to receive either gold standard prolonged exposure weekly for 15 weeks or 5 days of intensive treatment with the addition of three booster sessions dispersed 1, 2 and 4 weeks after completion of the massed treatment period.