A 15-year-old high school football player noticed asymmetry of his right shoulder and thought that he was losing muscle mass around his right shoulder over the previous 2-3 weeks. There was no history of pain, recent or past direct trauma or surgery of the neck or thorax. He was diagnosed with serologically confirmed infectious mononucleosis about 2 months earlier, and was recovering clinically. Physical examination was normal, except for significant asymmetry and winging of the right scapula, and electromyographic and nerve conduction studies revealed demyealination and axonal damage to the right long thoracic nerve and paralysis of serratus anterior, most likely caused by Epstein-Barr virus infection. Conservative treatment approach allowing continued activity and strengthening exercises resulted in full functional recovery over the next 6-8 months.