Background: The Insomnia Severity Index (ISI) is widely used in clinical assessments of insomnia in patients injured in high impact motor vehicle accidents (MVAs). This study examines the criterion and convergent validity of ISI on this clinical population. Method: De-identified archival data were available on 112 post-MVA patients (37 men, 75 women, mean age 38.8 years, SD=13.1). They completed the ISI as well as the Brief Pain Inventory, the Rivermead Post-concussion Symptoms Questionnaire, the Subjective Neuropsychological Symptoms Scale (SNPSS), Items 10 to 12 of the Whiplash Disability Questionnaire (ratings of depression, anger, and of anxiety), Whetstone Vehicle Anxiety Questionnaire, Driving Anxiety Questionnaire (DAQ), Steiner’s Automobile Anxiety Inventory, and some of them also completed the PTSD Checklist for DSM-5 (PCL-5). The ISI responses were also available from a community sample of 21 controls (10 men, 11 women, mean age of 39.2 years, SD=18.5). Results: The mean ISI total score of post-MVA patients (23.6, SD=13.1) was significantly higher than the one of the controls (6.0, SD=5.4) and significant between groups differences in the same direction were also observed on all 7 individual ISI items: the magnitude of these underlying relationships ranged from Pearson point biserial r of 0.68 to 87. The ISI total score also significantly correlated with ratings of post-MVA pain, depression, generalized anxiety, scores on measures of the post-concussion and whiplash syndrome, PTSD, and on Whetstone’s and DAQ measures of post-MVA driving anxiety. Discussion and Conclusions: The results show an excellent level of criterion and convergent validity of ISI for clinical assessments of insomnia in post-MVA patients.