Purpose To describe the profile and associations of anisometropia and aniso-astigmatism in a population-based sample of children MethodsThe NICER (Northern Ireland Childhood Errors of Refraction) study used a stratified random cluster design to recruit a representative sample of children from schools in NI. Examinations included cycloplegic (1% cyclopentolate) autorefraction and measures of axial length, anterior chamber depth and corneal curvature. ² tests were used to assess variations in the prevalence of anisometropia and aniso-astigmatism by age-group, with logistic regression used to compare odds of anisometropia and aniso-astigmatism with refractive status (myopia, emmetropia, hyperopia). Mann Whitney U-test was used to examine inter-ocular differences in ocular biometry. ResultsData from 661 white children aged 12-13 years (50.5% male) and 389 white children aged 6-7 years (49.6% male) are presented. The prevalence of anisometropia ≥1DS did not differ statistically significantly between 6-7-year-old (8.5%, 95% CIs 3.9-13.1) and 12-13-year-old children (9.4%, 95% CIs 5.9-12.9). The prevalence of aniso-astigmatism ≥1DC did not vary statistically significantly between 6-7-year-old (7.7%, 95% CIs 4.3-11.2) and 12-13-year-old children (5.6%, 95% CIs 0.5-8.1). Anisometropia and aniso-astigmatism were more common in 12-13-year children with hyperopia ≥+2DS. Anisometropic eyes had greater axial length asymmetry than non-anisometropic eyes; aniso-astigmatic eyes were more asymmetric in axial length and corneal astigmatism than eyes without aniso-astigmatism.ConclusionsIn this population there is a high prevalence of axial anisometropia and corneal/axial aniso-astigmatism, associated with hyperopia but whether these relations are causal is unclear. Further work is required to clarify the developmental mechanism behind these associations.