Background: Motor vehicle crashes cause significant morbidity and mortality annually. Seat belt use has partially been associated with a decreased risk of morbidity and mortality among those involved in motor vehicle crashes. Persons injured in motor vehicle crashes and not wearing seat belts have an increased risk of admission to trauma centers for motor vehicle crash-related injury. The purpose of this study was to measure changes in seat belt use after discharge among patients admitted to a Level I trauma center for injuries sustained in motor vehicle crashes. Methods: Patients admitted to a Level I trauma center for injuries sustained in motor vehicle crashes during 1998 were eligible for participation. A telephone interview was conducted with a random sample of 136 eligible patients regarding patterns of seat belt use before and after their collision. Demographic data and clinical characteristics were also collected. The frequency of seat belt use before and after crash involvement was compared for all patients and stratified by age, gender, race, and Injury Severity Score (ISS). Results: Slightly over half (54%) of patients reported "always" wearing a seat belt before their collision compared with 85% afterward. Younger age groups, male subjects, and whites had the largest increases in the frequency of seat belt use after collision (45%, 37%, and 44% increases, respectively). With respect to injury severity, the largest increase in the frequency of seat belt use was among those with ISS of 15 to 25 (82% increase). Significant concordance between patient- and emergency medical service-reported use of seat belts was observed. Among subjects reported by emergency medical service personnel to have been restrained, nearly 90% reported belt use at the time of the telephone interview. The most frequently cited occasion for failure to use seat belts (30%) was when taking short trips. Other reported reasons were forgetting to fasten belts (29%), discomfort (10%), being in a rush (8%), riding in the back seat (4%), and that seat belts were unnecessary when riding with a good driver (3%). Conclusion: Involvement in a motor vehicle crash results in increased seat belt use. Prevention efforts should be directed toward those patients who report infrequent use. Patient "converts" to seat belt use after collisions may be useful in public awareness and prevention campaigns.