Conjunctivitis is a frequent disease mostly caused by infections. In the case of a single-sided manifestation and additional resistance to anti-infective therapy a widespread differential diagnosis has to be considered. The case report of a 41-year-old patient suffering from unilateral persistent conjunctivitis is presented. Initially, there was no biomicroscopic evidence for a penetrated ocular foreign body. The unilateral, chronic and sectoral conjunctivitis was resistant to topical application of antibiotics and steroids. After repeated in-depth anamnesis the patient recalled an inconsiderable trauma while he was working on a building site. High-frequency ultrasound was used to locate an intrascleral foreign body 3 mm behind the limbus corneae while slit lamp biomicroscopy and ophthalmoscopy failed to detect the foreign material. The precise localisation provided by the 20-MHz sector scanner optimised the management of operative intervention and consequently led to an uncomplicated extraction of the bulbar foreign body.