Thirty-five patients having paretic vertical strabismus were evaluated to determine which clinical parameters could best be used to differentiate congenital or early onset vertical strabismus from late onset vertical strabismus. Those patients with late onset deviations had a higher incidence of diplopia, a higher incidence of cyclodeviation, a higher incidence of compensatory head posture, and manifested larger vertical deviations in the primary position. However, the degree of incomitance, the prevalence of amblyopia, and the level of stereoacuity, did not distinguish the two patient groups. Prism therapy was equally successful for both groups.