BACKGROUND AND PURPOSE: Prior annualized estimates of pediatric ischemic stroke incidence have ranged from 0.54 to 1.2 per 100,000 US children but relied purely on diagnostic code searches to identify cases. We sought to obtain a new estimate using both diagnostic code searches and searches of radiology reports and to assess the relative value of these 2 strategies. METHODS: Using the population of 2.3 million children (<20 years old) enrolled in a Northern Californian managed care plan (1993 to 2003), we performed electronic searches of (1) inpatient and outpatient diagnoses for International Classification of Diseases, 9th Revision codes suggestive of stroke and cerebral palsy; and (2) radiology reports for key words suggestive of infarction. Cases were confirmed through chart review. We calculated sensitivities and positive predictive values for the 2 search strategies. RESULTS: We identified 1307 potential cases from the International Classification of Diseases, 9th Revision code search and 510 from the radiology search. A total of 205 ischemic stroke cases were confirmed, yielding an ischemic stroke incidence of 2.4 per 100,000 person-years. The radiology search had a higher sensitivity (83%) than the International Classification of Diseases, 9th Revision code search (39%), although both had low positive predictive values. For perinatal stroke, the sensitivity of the stroke International Classification of Diseases, 9th Revision codes alone was 12% versus 57% for stroke and cerebral palsy codes combined; the radiology search was again the most sensitive (87%). CONCLUSIONS: Our incidence estimate doubles that of prior US reports, a difference at least partially explained by our use of radiology searches for case identification. Studies relying purely on International Classification of Diseases, 9th Revision code searches may underestimate childhood ischemic stroke rates, particularly for neonates.