Posttraumatic stress disorder (PTSD) occurs in response to an extreme traumatic stressor. It is estimated that as many as 1 million VA patients who were exposed to combat may have PTSD. VA patients with PTSD recently have been designated as a �Special Emphasis� population for which health care resources are allocated at approximately 14 times those allocated to patients without such chronic, complex, and intensive health care needs (i.e., �Basic Care� groups). Despite recognition of its substantial impact upon VA resources, the extent and means by which PTSD affects health and health care use remain unclear. This study seeks to enhance understanding of both the extent and means by which PTSD affects the health and the health care use of patients in VA ambulatory care. To meet this goal, we build on an ongoing HSR&D project, the Veterans Health Study (VHS), a prospective longitudinal study of 2,425 VA ambulatory care patients.