AIMS/BACKGROUND: The measurement of visual acuity is the most widely accepted indicator of amblyopia and is thought by some to be the only effective screening test. The aim of this study was to investigate the effectiveness of the traditional single optotype Sheridan-Gardiner test (SGT) in the measurement of visual acuity and the detection of amblyopia, compared with the log based linear format Glasgow acuity cards (GAC). METHODS: In the present study visual acuity was measured monocularly in 702 primary 1 schoolchildren using both acuity tests. RESULTS: A significant difference was found in the mean (SD) visual acuity measured with GAC (0.9 (0.08) modified logMAR) and SGT (1.13 (0.09) modified logMAR), df = 632, t = -59.08, p = 0.0001. The majority of children (89.3%) achieved visual acuities better than 6/6 in either eye when using the single optotype test. If the 95% confidence limits for a significant interocular difference in acuity are used as-criteria for the detection of unilateral amblyopia, GAC were found to be the most sensitive, correctly identifying 100%, while SGT identified 55% of the children with unilateral amblyopia. CONCLUSION: The results of this study highlight several problems with both the test format and testing procedure in the present school screening system.