Objective:To determine the incidence and types of childhood hypertropia in a defined population.Design:Retrospective population-based cohort.Participants:All pediatric(<19 years of age) residents of Olmsted County,Minnesota,diagnosed with vertical strabismus from January 1 1985,through December 31 1994.Methods:The medical records of all potential patients identified by the resources of the Rochester Epidemiology Project were reviewed.Main Outcome Measures:Incidence and types of childhood hypertropia.Results:Forty-two cases of childhood hypertropia were identified during the 10-year period,yielding an annual age-and gender-adjusted incidence of 12.9(95% confidence interval,9.0-16.9) per 100 000 patients younger than 19 years of age.This rate corresponds to a prevalence of approximately 0.26%,or 1 in 391,of all children younger than 19 years of age.Nearly three fourths(71.4%) of the children had a IVth cranial nerve palsy,primary inferior oblique overaction,Brown syndrome,or a vertical tropia in the setting of an abnormal central nervous system.Conclusions:The incidence rates for childhood hypertropia in this population-based study are higher than published reports of prevalence.Fourth cranial nerve palsy and primary dysfunction of the inferior oblique muscle were the most common forms of vertical strabismus in this population. Objective: To determine the incidence and types of childhood hypertropia in a defined population. Design: Retrospective population-based cohort. Participants: All pediatric (<19 years of age) residents of Olmsted County, Minnesota, diagnosed with vertical strabismus from January 1 1985, through December 31 1994. Methods: The medical records of all potential patients identified by the resources of the Rochester Epidemiology Project were reviewed. Main Outcome Measures: Incidence and types of childhood hypertropia. Results: Forty-two cases of childhood hypertropia were identified during the 10-year period, yielding an annual age-and gender-adjusted incidence of 12.9 (95% confidence interval, 9.0-16.9) per 100 000 patients younger than 19 years of age. This rate corresponds to a prevalence of approximately 0.26 %, or 1 in 391, of all children younger than 19 years of age. Nearly three fourths (71.4%) of the children had a IVth cranial nerve palsy, primary inferior oblique overaction, Brown syndrome, or a vert ical tropia in the setting of an abnormal central nervous system. Conclusions: The incidence rates for childhood hypertropia in this population-based study are higher than published reports of prevalence. Fourth cranial nerve palsy and primary dysfunction of the inferior oblique muscle were the most common forms of vertical strabismus in this population.