Otolaryngological complications occurred in 5 per cent of 467 patients with infectious mononucleosis admitted consecutively to the ENT-department during the period 1 April 1969 to 30 June 1981. Four patients had peritonsillar abscess and five patients developed potentially lethal obstruction of the upper airway. Such complications require intervention and, as the latter condition probably is the most frequent fatal complication of IM, we find it important that patients with even slight respiratory embarassment in infectious mononucleosis should be observed and treated in an ENT-department.