Abstract Purpose: To determine the prevalence and causes of decreased visual acuity (VA) Design: Population-based cross-sectional study. Participants: Multi-ethnic sample of children 30 to 72 months of age identified in Los Angeles. Methods: All eligible children underwent comprehensive ophthalmic evaluation including monocular VA testing, cover testing, cycloplegic autorefraction, fundus evaluation, and VA retesting with refractive correction. Decreased visual acuity was defined as presenting or best-measured VA worse than 20/50 in children aged 30 to 47 months and worse than 20/40 for children 48 months of age and older. The prevalence and causes of decreased VA were determined, for both presenting and best-measured VA, in the better seeing and the worse seeing eyes. Main Outcome Measures: Prevalence and causes of decreased vision. Results: Presenting VA was assessed in 1840 children and best-measured VA in 1886 children. Presenting VA was decreased in the worse eye of 4.2% of Asian and 3.6% of Non-Hispanic White (NHW) children. Close to one fourth of these cases had no identifiable etiology, and 81% of these resolved on retesting. Decreased presenting VA in the worse eye with an identifiable ophthalmic etiology was present in 3.4% of Asian and 2.6% of NHW children. Decreased presenting VA attributable to simple refractive error (myopia ≥0.5 diopters [D]; hyperopia ≥3.0 D; astigmatism ≥2.0 D or ≥1.5 D for children >36 months of age) was present in the worse eye of 2.3% of Asian children and 1.4% of NHW children, and in the better eye of 0.5% of Asian children and 0.3% of NHW children. Decreased best-measured VA attributable to a cause was present in the worse eye of 1.2% of both Asian children and NHW children, and in the better eye of 0.2% of Asian and 0.3% of NHW children. Amblyopia related to refractive error was the most common cause, and ten times as common as ocular disease. Severe VI was rare. Conclusion: Seventy percent of all decreased VA in Asian and NHW preschool children, and over 90% of decreased VA with an identifiable cause, is related to refractive error - either uncorrected refractive error or amblyopia due to refractive error.