Abstract. The Early vs. Late Infantile Strabismus SurgerY Study Group is a group of strabismologists and orthoptists who investigate whether early or late surgery is preferable in infantile strabismus, in a non-randomized, prospective, multi-centre trial. Infants between 6 and 18 months of age will receive a standardized entry examination and then be operated either before their see"Ond anniversary in clinics A, or between their 32nd and 60th month of age in clinics B. The children will be evaluated at age six. After completion of the study, the two groups can then be compared regarding degree of binocular vision, angle of strabismus and visual acuity of the worse eye relative to the better.