Posttraumatic stress disorder (PTSD) is common worldwide, with prevalence rates ranging from 1% to nearly 40%, depending on the population studied. The disability and natural course of PTSD in psychiatric patients have been well characterized. However, even though the primary care setting has been described as the "de facto mental health care system," surprisingly little is known about PTSD in primary care. Available data from primary care clinics in the United States and Israel suggest that PTSD may be as prevalent in this setting as has been reported in large epidemiologic studies. Patients may be unlikely to endorse traumatic experiences or may not consider them related to their current psychological problems. The prevalence of PTSD in primary care may indeed be higher than expected because of underreporting of domestic violence and other histories of trauma. Recognition of PTSD in primary care could be greatly improved if simple trauma histories were integrated into routine medical examinations. Primary care clinicians who maintain a high index of suspicion for PTSD in their patients with positive histories of trauma plus symptoms of depression or anxiety or other signs of psychological distress, suicidal thoughts or actions, alcohol or substance abuse, or excessive health care service utilization may increase the recognition rate of this disorder in their practices.