Post-traumatic stress disorder (PTSD) is caused by a traumatic experience often involving physical harm or the threat of harm or death. The emotional numbness and traumatic flashbacks symptomatic of PTSD interfere with everyday life for approximately 7.7 million adults. Besides prescription drug treatment, only exposure-based therapies, like prolonged exposure (PE) therapy, have been proved effective in treating PTSD. Interpersonal psychotherapy (IPT), which is not based on exposure, is effective in treating mood disorders, and pilot studies indicate it may also be effective in treating PTSD. IPT treats patients by helping them to improve their interpersonal functioning, as opposed to PE, which helps patients by guiding them to recreate traumatic memories in safe circumstances. This study will determine whether IPT is as effective as PE, the gold standard, in treating PTSD. Relaxation therapy, a commonly used control therapy for studies of PTSD, will be used for that purpose here.