The reliability of measurement of visual acuity by Teller's card was studied in young children under 18 months (3 to 42.5 months) to detect amblyopia. Repeated tests with Teller's cards were performed in children (two exams at least) divided into several pathologies. Visual acuity changed over time (in 2000 exams). Results were compared with those given by drawing tests (Pigassou) at two years and a half. Middle amblyopia occurred in 44%, low amblyopia in 40% and high amblyopia in 16%, as the results by Teller's cards were considered as normal. These results were found in 54% among the 2000 Teller's studied. Teller's test was thus shown to be unreliable to detect middle or low amblyopia. Objective réfraction is required.