Four hundred and twenty-five normal children in the age range 18 months to 4 1/2 years have been studied to determine the percentage of children according to age who can be expected to cooperate in a letter-matching vision test using a key card; to see whether the use of plastic letters to replace the key card lowers the age at which a letter-matching test can be achieved, and to show that children in this age range can do a vision test at 20ft (6m). Acceptance of occlusion using a patch occluder was also examined. The study shows that children effectively begin to cooperate in letter-matching vision tests using a key card from 33 months of age. Replacement of the key card with plastic letters lowers the age to 30 months and thereafter there is a gain of some 25% of children up to 39 months. All children were successfully tested at 20ft (6m). The most difficult age for occlusion was in the age group around the second birthday.