Aims To describe the prevalence of refractive error (myopia and hyperopia) and visual impairment in a representative sample of white school children. Methods The Northern Ireland Childhood Errors of Refraction study, a population-based cross-sectional study, examined 661 white 12e13-year-old and 392 white 6-7-year-old children between 2006 and 2008. Procedures included assessment of monocular logarithm of the minimum angle of resolution (logMAR), visual acuity (unaided and presenting) and binocular open-field cycloplegic (1% cyclopentolate) autorefraction. Myopiawas defined as -0.50DS or more myopic spherical equivalent refraction (SER) in either eye, hyperopia as >=+2.00DS SER in either eye if not previously classified as myopic. Visual impairment was defined as >0.30 logMAR units (equivalent to 6/12).Results Levels of myopia were 2.8% (95% CI 1.3% to 4.3%) in younger and 17.7% (95% CI 13.2% to 22.2%) in older children: corresponding levels of hyperopia were 26% (95% CI 20% to 33%) and 14.7% (95% CI 9.9% to 19.4%). The prevalence of presenting visual impairment in the better eye was 3.6% in 12-13-year-old children compared with 1.5% in 6-7-year-old children. Almost one in four children fails to bring their spectacles toschool.Conclusions This study is the first to provide robust population-based data on the prevalence of refractive error and visual impairment in Northern Irish schoolchildren. Strategies to improve compliance with spectacle wear are required.