Patient has longstanding right IV nerve palsy with left head tilt. When RE is covered no movement of LE; when LE is covered no movement of RE (with head tilt). Alternating cover testing right hyperphoria of approximately 8Δ with good recovery. Without head tilt the right hyperphoria becomes more apparent. During oculomotility testing, with patient looking down and to the left, LE is covered the RE moves down to take up fixation; this is more obvious with alternating cover test. Indicates that the right superior oblique is underacting. Patient tilts head to the left there is virtually no eye movement on cover-uncover or alternating cover tests. Patient tilts head to the right the RE elevates without covering. Elevation is more apparent with alternate cover testing. Positive Bielschowsky test, confirms right underacting superior oblique.