Among motor vehicle crash head injuries, subdural hematomas (SDH) are both frequent and lifethreatening, especially for older occupants. Previous research on the mechanism of injury and on the increased vulnerability of older individuals to SDH has focused on the failure of bridging veins, which are one possible source of subdural bleeding. For all age groups, the injury mechanism and injury tolerance for SDH as a result of other bleeding sources has not been addressed. In the current study, two US crash databases were used to compare crash and injury characteristics for SDH cases in different age groups, with a focus on the original source of bleeding. Review of cases from the Crash Injury Research and Engineering Network (CIREN) database showed that both bridging veins and bleeding sources other than bridging veins are responsible for SDH among crash occupants in all age groups. Analysis of weighted data from the National Automotive Sampling System Crashworthiness Data System (NASS CDS) showed that the frequency of isolated SDH increases with age, potentially reflecting an increase in the frequency of SDH caused by bridging vein bleeding, particularly in frontal crashes and among women. SDH accompanied by brain contusions or other potential bleeding sources on the surface of the brain are also common, especially in side impacts and among occupants younger than 70. These cases potentially represent injuries where subdural bleeding came from sources other than bridging veins. Improved definition of SDH injury tolerance for all adult occupants will require a better understanding of the mechanism of injury from sources other than bridging veins, but determination of SDH injury tolerance for older occupants should focus on evaluation of the increasing risk of bridging vein failure with age.