We report a case of sudden hearing loss in a patient with acute exudative tonsillitis, occurring 15 minutes after the intramuscular administration of penicillin. Audiological evaluation documented a profound sensorineural hearing loss of the cochlear type. The mechanism of the hearing loss was probably an immediate hypersensitivity (type I) allergic drug reaction. Penicillin is used frequently for the treatment of several infections. Allergic reactions to penicillin are well known and include urticaria, maculopapular exanthems, angio-oedema, bronchospasm and anaphylaxis, but sudden hearing loss has never been recorded.