Strabismus surso-adductorius is a frequent unilateral or bilateral eye movement disorder. Its clinical features include eye elevation with concomitant vertical deviation in adduction, an abnormal head posture from which the patient is unaware (head turned and tilted towards the healthy side), a moderate subjective excyclotorsion, and a positive Bielschowsky head tilt test. Despite its anglo-saxon denomination as "congenital fourth nerve palsy", it is not a paretic disorder. Strabismus surso-adductorius differs from fourth nerve palsy both by etiology and by symptoms. A proper diagnosis is important as neuroradiological examination is mandatory in cases of acquired non-traumatic fourth nerve palsy, whereas decompensated strabismus surso-adductorius can be operated on without any further investigations. Early on, the oculomotor disorder is often well compensated and it does manifest at the adult age. Asthenopia and intermittent vertical diplopia appear as the fusional mechanisms fade out. The best surgical technique for strabismus surso-adductorius is an inferior oblique weakening procedure. In severe cases a combined shortening of the superior oblique tendon may be necessary.