Although post-traumatic stress disorder (PTSD) is a debilitating anxiety disorder that may cause significant distress and increased use of health resources, the condition often goes undiagnosed. The lifetime prevalence of PTSD in the United States is 8 to 9 percent, and approximately 25 to 30 percent of victims of significant trauma develop PTSD. The emotional and physical symptoms of PTSD occur in three clusters: re-experiencing the trauma, marked avoidance of usual activities, and increased symptoms of arousal. Before a diagnosis of PTSD can be made, the patient's symptoms must significantly disrupt normal activities and last for more than one month. Approximately 80 percent of patients with PTSD have at least one comorbid psychiatric disorder. The most common comorbid disorders include depression, alcohol and drug abuse, and other anxiety disorders. Treatment relies on a multidimensional approach, including supportive patient education, cognitive behavior therapy, and psychopharmacology. Selective serotonin reuptake inhibitors are the mainstay of pharmacologic treatment.