Twelve patients between 14 and 26 years of age, ten with infectious mononucleosis and two with Guillain-Barré syndrome, all of whom were suspected of having had Epstein-Barr viral infection, developed daytime sleepiness. The daytime somnolence was confirmed objectively by polygraphic monitoring seven weeks to 4 1/2 months after the onset of clinical symptoms. The patients have been followed up for three to 12 years. None has had any other neurologic sequelae, but all have disabling daytime sleepiness. Treatment brings only mild relief. A retrospective chart study of 35 patients with infectious mononucleosis did not identify, on the basis of initial clinical symptoms, those patients who developed chronic impairment.