In longitudinal studies of traumatic stress, it is particularly important to examine the data for any differences between those who drop out and those who continue to participate, because reluctance to participate might reflect symptoms of avoidance frequently seen in posttraumatic stress disorder (PTSD). However, whether those who drop out are at high risk of PTSD remains unclear. Over a 25-month period, 188 consecutive patients with motor vehicle accident (MVA)-related injuries admitted emergently were enrolled and followed for 4 to 6 weeks. Baseline characteristics were compared between subjects who did and did not participate in the follow-up study. At 4 to 6 weeks, 66 (35.1%) of the participants had dropped out. Bivariate analyses revealed that those who dropped out were likely to be men, alcohol drinkers, smokers, and unconscious just after MVA and to have fewer years of education, less severe injuries, less posttraumatic symptoms, and lower cooperativeness as assessed by the Temperament and Character Inventory. Logistic regression analysis revealed that male sex, unconsciousness during MVA, low cooperativeness, and less severe injuries were significant predictors of dropout. The literature says that male sex and unconsciousness just after MVA might be protective factors against MVA-related PTSD, whereas low cooperativeness is a risk factor for general mental problems. To summarize, it is expected that those who drop from the follow-up are unlikely to have MVA-related PTSD, but might have mental problems independent of injury.