This article reports the first estimates of overweight prevalence in Chicago children entering school (aged 3-5 years). Chicago data are compared with those from the National Health and Nutrition Examination Survey (NHANES) and the Early Childhood Longitudinal Study (ECLS). Data were from 2 separate convenience samples of children aged 3-5 years attending either 18 Chicago Public Schools or 10 Chicago Catholic School pre-K programs (n = 1517). Data were taken from students' Certificate of Child Health Examination (CCHE), completed by a health professional. Overall, the prevalence of overweight subjects (body mass index >or=95th percentile) was 24%, more than twice that of the national prevalence of 10% for 2- to 5-year olds documented by NHANES (1999-2002) and 3 times that of the 1998-1999 ECLS prevalence estimate of 8% for 5- to 7-year olds in the Midwest region. The data reported here document that nearly one quarter of children entering school in Chicago are already overweight. This clearly establishes a need for local schools to develop protocols and procedures to support the physical and mental health needs of affected and at-risk children. The findings also make it plain that ongoing weight status monitoring is needed and that current plans to implement this should go forward.