A case of brachial plexus neuropathy (BPN) associated with infectious mononucleosis (IM) in a 13 year old boy is presented. The essential clinical picture of BPN is discussed and the main points are: acute pain at onset across the top of the shoulder-blade followed by weakness and atrophy on muscles supplied in most cases by superior brachial plexus. The prognosis is good and most patients began to note improvement within months. The main neurological complications of 1M are lymphocytic meningitis, peripheral neuropathy and encephalomyelitis and they can occur without any classical findings as pharyngitis, adenopathy or splenomegaly. The association of BPN and IM was described previously in only 7 cases and as the present case all of them had a complete recovery.