This population-based retrospective cohort study identified all Olmsted County, MN residents with any diagnosis indicative of potential traumatic brain injury (TBI) during the years 1985 to 2000. The complete community-based medical records of a random sample (n = 7,175) were reviewed to confirm and characterize the event, and to determine vital status through 2002. The review identified 1,448 confirmed incident cases; 164 (11%) were moderate to severe; 1,284 were mild. The estimated 30-day case fatality rate was 29% for moderate to severe cases and 0.2% for mild cases. Comparison of observed mortality over the full period of follow-up with that expected revealed a risk ratio (95% CI) of 5.29 (4.11–6.71) for moderate to severe cases and 1.33 (1.05–1.65) for mild cases. Proportional hazards modeling showed the adjusted hazard of all-cause mortality for moderate to severe cases relative to mild cases was 5.18 (3.65–7.3) within six months of the event and 1.04 (0.57–1.88) for the remaining follow-up period. This analysis indicates that persons who experience mild TBI exhibit a small but statistically significant reduction in long-term survival compared to the general population. The case fatality rate for persons with moderate to severe TBI is very high, but among six-month survivors, long-term survival is similar to that for persons with mild TBI.