The purpose of this study is to determine whether 60-minute sessions of prolonged exposure (PE) are as effective as the standard 90-minute session for treating posttraumatic stress disorder (PTSD). Participants will include patients ages 18 or older with a current diagnosis of PTSD who are seeking treatment in our clinic. Patients who have current substance dependence, psychosis, and suicidal ideation with intent and plan may not be suitable for receiving PE and may be offered another treatment or referred to a different treatment center. Participants will be randomized to receive either the 90- minute or 60-minute PE session. A blind evaluator will assess for pre-treatment, post-treatment, and follow-up levels of symptom severity using the PTSD Symptoms Scale Interview (PSS-I). Participants will attend weekly treatment sessions with any of our faculty members and will complete self-report measures at every session (see below).