The cerebral trunk infection by Listeria monocytogenes is a very rare process, with only 24 described cases and only 2 in immunodepressed patients. We present a new case of cerebral trunk encephalitis caused by Listeria in a patient diagnosed of Polyarteritis nodosa on steroids and cyclophosamide treatment and fatal evolution, and whose hemoculture, spinal fluid culture and cranial CT scan did not contribute to the diagnosis. A necrotizing lesion in the protuberance with mesencephalic extension was observed in the necropsy study in which intra- and extracellular Gram + and silver positive bacilli were detected. Since L. monocytogenes was suspected as the causative agent, indirect immunofluorescence was performed on histological preparations with hyperimmune anti-L. monocytogenes serum giving a clearly positive result. In our case, this technique permitted the etiological diagnosis in the absence of positive cultures.