**2.1 Participants**

Participants were selected from the Tachikawa Cohort of Motor Vehicle Accidents study conducted at the National Disaster Medical Center in Tokyo, Japan (Matsuoka et al., 2009). The inclusion criteria in the present study were as follows: 1) motor vehicle accident-related severe physical injury causing a life-threatening or critical condition; 2) age between 18 and 69 years; and 3) native Japanese speaking ability. The exclusion criteria were the following: 1) diffuse axonal injury, brain contusion, and subdural and subarachnoidal bleeding detected by either computed tomography or magnetic resonance imaging or both (with the exception of concussion), because the presence of traumatic brain injury creates considerable difficulties when assessing psychological responses to injury; 2) cognitive impairment, defined as a score of <24 on the Mini Mental State Examination; 3) currently suffering from schizophrenia, bipolar disorder, drug dependence or abuse, or epilepsy before the accident; 4) marked serious symptoms such as suicidal ideation, self-harm behavior, dissociation, or a severe physical condition preventing the patient from tolerating the interview; and 5) living or working at a location more than 40 km from the National Disaster Medical Center.

The above-mentioned study was conducted between 30 May 2004 to 8 January 2008, and the present study is part of that larger study. Patients with motor vehicle accident-related physical injury were consecutively admitted to the intensive care unit of the National Disaster Medical Center between 18 August 2005 and 8 January 2008. Of the 221 patients who met the inclusion criteria, 189 agreed to participate in the study. Fifty-nine patients were excluded because their peritraumatic distress could not be assessed due to memory loss. Ultimately, 130 patients participated in this study.
