To report the incidence and demographics of childhood ptosis diagnosed over a 40-year period in a well-defined population. Retrospective, population-based cohort study. Patients (<19 years) diagnosed with childhood ptosis and residents of Olmsted County, Minnesota, from January 1, 1965, through December 31, 2004. The medical records of all potential patients identified by the Rochester Epidemiology Project were reviewed. Calculated annual age- and gender-specific incidence rates and demographic information. A total of 107 children were diagnosed with ptosis during the 40-year period, yielding an incidence of 7.9 per 100000 younger than 19 years (95% confidence interval [CI], 6.4-9.5). Ninety-six (89.7%) of the 107 had congenital-onset disease. Eighty-one (75%) of the 107 had simple congenital ptosis, yielding a birth prevalence of 1 in 842 births. A family history of childhood ptosis was present in 12% of queried patients with simple congenital ptosis. Three (4%) of the simple congenital ptosis cases were bilateral and 55 (68%) of the unilateral cases involved the left upper eyelid (95% CI, 57%-78%; P<0.001). Childhood ptosis was diagnosed in 7.9 per 100000 patients younger than 19 years (95% CI, 6.4-9.5). Simple congenital ptosis was the most prevalent form, occurring in 1 in 842 births, and was significantly more likely to involve the left side. The author(s) have no proprietary or commercial interest in any materials discussed in this article.