PURPOSE: To describe the occurrence and heritability of astigmatism in a population-based sample of Norwegian twins. METHODS: Self-reported history of astigmatism based on questionnaire responses was used to estimate the prevalence and incidence rates of astigmatism from birth through 31 years in 8,045 twins. Kaplan-Meier analysis and log-binomial regression were used to study the conditional and relative risk of astigmatism in twin pairs by sex and zygosity. Tetrachoric correlations and structural equation models were applied to estimate the genetic and environmental sources of variations in liability for astigmatism. RESULTS: Altogether 21.1% of males and 29.3% of females (p