The authors examined the efficacy of Prolonged Exposure (PE) therapy in Japanese patients with posttraumatic stress disorder (PTSD). Twenty-four patients (21 women, 3 men) with PTSD due to mixed trauma were randomly assigned to the PE group (PE with or without treatment as usual [TAU]) or the control group (TAU) only. The control group received PE after a 10-week period. Intention-to-treat analysis showed the PE group achieved significantly greater reduction than the control group at posttreatment in either PTSD or depressive symptoms. The control group had significantly decreased symptom severity after PE treatment. Symptom levels of 19 PE completers in the both groups remained low in 12-month follow-up assessments. The study's findings will promote the future dissemination and implementation of evidence-based treatment for PTSD in non-Western settings.