Objective To examine the costs of road traffic injuries (RTIs) in Vietnam and factors associated with increased costs. Method RTI data were collected in a prospective cohort study on the impact of injuries in Vietnam. Participants were persons admitted to the Thai Binh General Hospital because of RTI. All costs incurred by participants and their family members during hospitalisation were collected, including direct medical costs, direct nonmedical costs and indirect costs. Generalised linear models were employed to examine predictors of increased costs including demographic and injury context characteristics. Results Each RTI hospitalisation costs the patient and family on average US$363 or 6 months of average salary. Income, injury severity, principal region of injury and length of hospital stay were statistically significant predictors of increased costs; age, gender, occupation and road user group were not. After controlling for injury characteristics and income, participants with principal injuries to the lower extremities had a cost 1.28 (95% CI 1.07 to 1.54) times higher than those with principal injuries to the face. Analyses of motorcycle-related RTIs with principal injury to the head also showed increased costs among those without a helmet (1.41 times higher, 95% CI 1.17 to 1.71). Conclusions RTIs can cause a substantial economic burden to the patient and family. During hospitalisation on average, an RTI would cost approximately 6 months of salary. In addition to interventions to decrease the risk of RTIs, those reducing the severity, such as wearing a motorcycle helmet, should be enforced to minimise the economic and health consequences of injury.