We report an unusual case of simultaneous occurrence of peritonsillar abscess due to Streptococcus viridans with an aneurism of the internal carotid artery. Inflammation caused by the abscess was so marked that its operative treatment unexpectedly uncovered the aneurism. We believe that the aneurism probably resulted from lymphatic spread of inflammation. Although rarely occurring, such aneurisms are clinically very important: the danger of misinterpreting symptoms as those of a peritonsillar or retropharyngeal abscess is great, as is that of bleeding caused by incising an abscess overlying the aneurism. This is particularly important in children due to the frequent occurrence of tonsil infections in this age group. The differential diagnosis of posterior or lateral pharyngeal swellings in children does not generally include aneurisms since these seldom occur in children and are overall rare in occurrence.