Evaluating the vision of retarded children is important, but it is often very difficult because the children cannot cooperate with the usual symbol recognition acuity tests. A total of 33 unselected children with Down's syndrome were tested with a standard symbol recognition test (Osterberg) and with a new test using acuity cards, which contain black and white square-wave gratings. The acuity cards were found to be useful for testing this group, both in terms of the children's ability to cooperate with the test and the test-retest variability of the results. Although eyes with strabismic amblyopia showed better acuity for gratings than for symbols, there was general agreement with symbol recognition acuities of the same patients. Acuity cards were also used to test 19 severely retarded young children with cerebral palsy, who were unable to be tested with symbols. Acuity card tests were almost always possible with this group, too, though test-retest variability was greater for these children. Acuity card testing can provide useful information about the visual acuity of many retarded children in less than 10 minutes.