Children born to myopic parents have the longer eyes associated with myopia even before myopia has set in. Juvenile onset myopia usually develops between the ages of 8 and 14, and affects 15% of all children by the time they reach 15. The refractive error, corneal curvature, crystalline lens power and axial ocular dimensions of 716 first, third and sixth grade students were measured. Of these students, 180 had two parents with myopia, 316 had one parent with myopia and 220 had no myopic parents. Of those with two myopic parents, 12.2% were myopic. Of those with one myopic parent, 8.2% were myopic and of those without a myopic parent, 2.7% were myopic. Non-myopic children with two myopic parents had longer eyes, including longer anterior and vitreous chambers, and weaker crystalline lens power than the other children. Those with one myopic parent had longer eyes than those with no myopic parents., Objective. - To evaluate whether eye size and shape are different in children based on their parental history of myopia. Design. - A community-based cohort study of schoolchildren (aged 6 to 14 years), the Orinda (Calif) Longitudinal Study of Myopia. Setting. - Four campuses of the Orinda Union School District, a predominantly white, high socioeconomic status community. Participants. - A cross-sectional volunteer sample of 716 children (662 nonmyopic) in the first, third, and sixth grades in 1989,1990, and 1991. All children in those grades were eligible for inclusion in the study. Intervention(s). - None. Main Outcome Measures. - Refractive error (measured by autorefraction), corneal curvature (measured by photokeratoscopy), crystalline lens power (measured by video phakometry), and axial ocular dimensions (measured by ultrasonography). Results. - With prevalent cases of myopia excluded and grade in school and "near work" controlled for, children with two myopic parents had longer eyes and less hyperopic refractive error (analysis of covadance, P [less than or equal to] .0l) than children with only one myopic parent or no myopic parents. A model incorporating parental history is only improved by the addition of near work for the prediction of refractive error. Conclusions. - Even before the onset of juvenile myopia, children of myopic parents have longer eyes. These results suggest that the premyopic eye in children with a family history of myopia already resembles the elongated eye present in myopia.