A review of a class of head injured patients who display a lucid interval before onset of coma which defines an important category of brain trauma. A hypothesis explaining the mechanism of this syndrome (T.A.D.D.) is developed on the basis of in-vitro testing of venous and arterial responses to tensile strains and the relevant clinico-pathologic data are reviewed. Venospasm is proposed as the cause of secondary ischemic hypoxia associated with a high incidence of acute subdural hematomas and brain swelling. Suggestions for further testing of this hypothesis and implications for preventive management are discussed.