Objective. To estimate the prevalence of undetected vision abnormalities among preschool children. These problems can lead to a variety of adverse consequences, the most serious being amblyopia, wich becomes irreversible after the fifth year without treatment. In some other countries there are screening programs to identify visual abnormalities among four year old children. In our country there are no studies about the prevalence of this problem among preschool children. Material and methods. A total of 39 preschool children were randomly selected from 93 kindergarten schools in a District of Mexico City. In total, 343 children were screened for ophthalmologic abnormalities by two pediatricians who were previously trained for this purpose. Results. Fifty seven (16.6%) children failed the visual acuity testing: four had strabismus, four cataract, and one glaucoma, and 48 refraction defects. Only 13 (3.8%) had been previously examined, ever. Conclusions. We consider that the prevalence found supports the recommendation to promote ophthalmologic examining as part of the routinary child care practice and to perform preventive preschool screening.