In spite of the general clinical uniformity of Wallenberg's syndrome (WS), individual patients present with a slightly different clinical picture, and detailed studies with magnetic resonance imaging (MRI) show differences in the topography of the brain stem lesion. Neurophysiological characterization of the lesion in WS has been known for a long time, but there are no studies on the possible correlation between lesion topography and neurophysiological deficit. Assuming that afferents from the three branches of the trigeminal nerve reach different parts of the trigeminal nuclei, we examined the possible correlation between the lesion topography assessed by the MRI and the neurophysiological deficit, assessed by studying the brain stem reflexes in patients with WS within 2 weeks after stroke. Neurophysiological abnormalities were always located in the afferent branch of the reflexes examined, but not all patients exhibited abnormalities in all responses. The ophthalmic branch was involved in 92.8% of patients, and the mandibular branch in 57.1 % of patients. The patients with MRI lesions located in the lower medulla had normal responses with infraorbital or mental nerve stimulation. The results of this neurophysiological study confirm the heterogeneity of WS. Whether the neurophysiological identification of different subgoups of patients is relevant for clinical outcome needs further studies. 0 1996