The Early vs. Late Strabismus Surgery Study Group is a group of strabismologists and orthoptists who wish to investigate whether early or late surgery is better in cases of infantile strabismus, in a controlled, multicentre, matched-pair trial: All infants will receive a standardized entry examination at age one and then be operated either before their second birthday in clinics A, or between the 32nd and 60th month of age in clinics B. The children will be evaluated at age six. After completion of the study, one child will be selected from each group, to form a pair of children with the same entry examination, who were operated early or late. Successive pairs will be generated so that finally two smaller groups with the same entry examinations are formed. These two groups can then be compared regarding degree of binocular vision, angle of squint and visual acuity of the worse eye. It was decided to perform first a pilot-study of the examination of infants age nine to 15 months, because we wanted to know to what degree of accuracy infants age one can be examined and what parts of the examination can be carried out most reliably and can therefore be used as parameters to match the pairs in the main study. 190 Children were each examined by three examiners on one day, according to a standardized examination sheet, and differences were quantified. We found that the angle of squint could be measured with reasonable precision: The largest difference between any two of the three measured angles averaged at 6.5″. The angle did not increase when the infant was examined a second or third time. Variability of the angle, vertical divergence and up-/downshoot-in-adduction could not be assessed reliably. On the other hand, restriction of abduction (in a 4-class scale) could be measured with adequate precision: in 58% there were no differences at all between the three examinations. The degree of amblyopia could be well assessed by observing the fixation pattern by means of direct fundoscopy with a fixation mark or by comparing corneal reflexes during monocular fixation. Using a 3-class scale, there were no differences at all between the three examinations of the three orthoptists in 94% of the cases. In addition, amblyopia could be accurately assessed by observing the fixation behaviour of the child with either eye, in a 5-class scale.