In both groups, the vertical deviation showed a great dispersion. In patients without HMOS (group 2), vertical deviation in adduction did not exceed 15°. Patients with HMOS (group 1) do not show the typical features of a later acquired trochlear palsy due to an early developed compensating innervation. A vertical deviation in adduction of more than 15°, increasing excyclodeviation towards downgaze in all three planes (sagittal plane, adduction and abduction) and Knapp's class II are relatively reliable predictors of a hypoplasia of the SOM. An internationally uniform term for this group of patients, such as superior oblique weakness or superior oblique hypotrophy, would be desirable.