In fifteen cases of brain stem lesions caused by basal tumours, operations, injuries, arteriosclerosis or multiple sclerosis, a paradoxical activity of jaw closing muscles during jaw opening was observed. This inverse activity was the cause of severe trismus in 8 cases. It is the manifestation of a faulty programming of jaw movements in the trigeminal motor area in brain stem. This distinct new brain stem syndrome can only be detected by EMG investigation. EMG findings, pathological background, outcome and treatment possibilities are discussed in this paper.