CASEs of primary brain stem lesion following closed head injury, verified on CT scan, have been increasingly reported recently. However, most of them have supratentorial lesions in addition to brain stem, resulting in a poor outcome. In this paper, a case of localized brain stem hematoma following closed head injury, is reported based on CT findings. A 26-year-old man slipped down on his back and hit the right occipital area with following loss of consciousness for several minutes. Since then, he continued to have gait disturbance and visited our hospital on the next day. On admission, he presented truncal ataxia, left trochlear nerve palsy, right Horner's syndrome and left hemihypesthesia. CT scan revealed a small hematoma localized to the right midbrain tegmentum at the level of inferior colliculus, well correlating with his clinical presentation. He gradually improved on conservative treatment and returned to his former work one month after the accident. This case suggests that there may be cases of primary brain stem injury with no other intracranial lesion based on CT findings and with a good prognosis. Shear strain is said to be a probable mechanism for explaining brain stem injury, which is usually combined with other parenchymal lesions. In our case, however, more focal factor--nervous and/or vascular compression against the tentorial edge--is suspected for producing this localized midbrain lesion.