Since the etiology of traumatic brainstem lesions has not been completely clarified we tried to analyse this question from autopsy material with special consideration of the effects of rotational acceleration of the head resulting from an impact force. Seventeen cases without a skull fracture were investigated. We considered the tentorium as a suitable indicator for possible relative movements occuring between cerebral hemispheres and the cerebellum as well as the brainstem since, because of its attachment to the skull, it follows all excursions of the skull directly and simultaneously. The brain, however, due to its inertia, remains stationary in the beginning so that injuries may occur on the structures mentioned. The results of the autopsies confirmed these assumptions, for both uni- and bilateral tears as well as hemorrhages in the tentorium cerebelli were found. Associated were subarchnoid hemorrhages over the upper vermis and thin layered hematomas between the tentorium and the cerebellum. In all cases the brainstems showed both microscopic and macroscopic hemorrhages of the isthmus cerebri and in some cases of the cerebral peduncles and the cerebellar brachia conjunctiva. These hemorrhages are attributed to shearing forces and tension arising from movements between tentorium and the surrounding brain components. The mechanical effects on the brainstem and its surrounding structures at various rotational accelerations of the head (along transversal, vertical and sagital axis) are discussed. The combination of brainstem lesions with injuries of the tentorium suggests that the brainstem injury was a primary result of the impact. Finally the possible significance of rotational acceleration for brainstem injuries caused by whiplash mechanism of the head in connection with improperly used automobile safety belts or inadequate headrests is discussed.