Two hundred and four cases of in-patient admission with isolated, unilateral peritonsillar abscess over the three-year period 1999–2001 were reviewed retrospectively. One hundred and fifty-one patients had been screened for infectious mononucleosis (IM) using the heterophile antibody screening test. Of these 142 (94 per cent) tested negative and nine (six per cent) positive. There were no IM-typical clinical or haematological signs in any of the IM positive patients to facilitate the prediction of the diagnosis. Due to the comparatively high prevalence of positives, the low cost of screening, the lack of predictive signs and the diversity of potential complications of IM, routine screening in all patients presenting with peritonsillar abscess is recommended.