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Background Concussion is an acute injury that may contribute to short-term limitations and potential long-term risks.Objective To test whether a past concussion is associated with the risk of a subsequent serious motor vehicle crash.Design Population-based longitudinal cohort analysis.Setting Ontario, Canada, from 1 April 2002 to 31 March 2022 (178 emergency departments).Patients Adults diagnosed with a concussion (cases) or an acute ankle sprain (controls), excluding individuals with a disqualifying illness (blindness, dementia, delirium), severe cases resulting in hospitalisation or those who died within 90 days.Primary measure Subsequent motor vehicle crash requiring emergency medical care.Results We identified 3 037 028 patients, including 425 158 with a concussion and 2 611 870 with an ankle sprain. A total of 200 603 patients were injured in a subsequent motor vehicle crash over a median follow-up of 10 years, equal to an absolute risk of 1 in 15 patients (6.64 per 1000 patient-years). Patients with a concussion had a 49% higher motor vehicle crash risk compared with those with ankle sprain (adjusted relative risk=1.49, 95% CI 1.47 to 1.50, p<0.001). The increased risk was particularly high in the early weeks after a concussion, remained independent of other observed risk factors, applied to diverse clinical groups and was further accentuated after repeated concussions. The risk extended across a spectrum of crash severity, was accentuated for single-vehicle events, replicated in analyses with artificial intelligence methods adjusting for confounding and remained distinct from the risks of other unrelated medical emergencies.Conclusions This study suggests a significant increased risk of a motor vehicle crash after a concussion that may justify a safety warning from clinicians.