Thirteen patients with Wallenberg's lateral medullary syndrome (WLMS) were studied. Clinical and magnetic resonance imaging (MRI) evidence demonstrated infarction in the dorsolateral medulla which produced loss of pain and temperature sensation on one side of the face ipsilateral to the lesion in seven patients. However, in another six patients, the infarction in a similar location produced the same sensory loss on the contralateral face. This is the first report of an analysis comparing these two conditions in WLMS patients, confirmed by MRI. The finding of normal blink reflex and somatosensory evoked potentials (stimulation on median nerve) in the two groups of patients may indicate that the impulses travel along the central pathways of touch, vibration and joint position sensation instead of the pathways for pain and temperature, because the patients had normal sensation of touch, vibration and joint position but impairment of pain and temperature sensation. In addition, it is suggested that the pathways of late blink reflex (R2) pass through the medial lemniscus in the ventromedial medulla instead of the spinal trigeminal tract in the dorsolateral medulla. Further, the observation of the much longer lantencies (about 29 ms) of the normal R2 raises the possibility that the impulses may travel along the longer pathways through the opposite medial lemniscus and up to the thalamus or cortex where they project to bilateral motoneurons of the orbicularis oculi muscles. Although the alternative of R2 travelling only by the shorter pathways through the brain stem is not excluded, this is not supported by current data.