The natural history of Chiari I malformation in children remains unclear. A population-based retrospective cohort study was therefore conducted. Radiology reports from all head and spine magnetic resonance imaging scans (n = 5248) performed among 741,815 children under age 20 within Kaiser Northern California, 1997-1998, were searched for Chiari I. Medical records and imaging studies were reviewed to determine clinical and radiographic predictors of significant neurologic symptoms, defined as moderate to severe headache, neck pain, vertigo, or ataxia. The 51 patients identified with Chiari I represented 1% of the children who had head or spine magnetic resonance imaging scans performed during the study period. Headache (55%) and neck pain (12%) were the most common symptoms. Syringomyelia was present in 6 patients (12%) at initial diagnosis; no new syrinxes developed during follow-up. Older age at time of diagnosis was associated with increased risk of headache (odds ratio OR = 1.3, 95% confidence interval CI = 1.1-1.5) and significant neurologic symptoms (OR = 1.2, 95% CI = 1.04-1.4). Chiari I, an underrecognized cause of headaches in children, is also frequently discovered incidentally in children without symptoms. Larger and longer-term studies are needed to determine the prognosis and optimal treatment of pediatric Chiari I.