Children with a chronic health condition have long been considered at excess risk for psychosocial morbidity. Despite an increasing prevalence of chronic childhood conditions and heightened concerns for the quality of life of the chronically ill, population-based studies of behavior problems among children with chronic physical conditions are rare. Findings on the epidemiology of behavior problems in a nationally representative sample of 11,699 children and adolescents aged 4 to 17 years in the United States are reported. Data included a 32-item parent-reported behavior problem index, measures of chronic childhood conditions, measures of school placement and performance, and sociodemographic variables. Analyses confirmed that chronic physical conditions were a significant risk factor for behavior problems, independent of sociodemographic variables. Among children these differences were observed across all subscales; among adolescents the largest differences were found for the Depression/Anxiety and Peer Conflict/Social Withdrawal subscales. Rates of extreme behavior problem scores (those in the top 10th percentile) were 1.55 times higher among children with a chronic health condition compared with children without a chronic condition (95% confidence interval 1.29 to 1.86). These independent odds were lowered to 1.44 when covariates for confounding were introduced via a multivariate logistic regression. Other independent risks included the absence of either biologic parent (odds ratio 2.05), male gender (1.53), low vs high family income (1.30), low vs high maternal education (1.51), and young vs old maternal age at childbirth (2.57). Chronic health conditions were also a major risk factor for placement in special education classes and having to repeat grades. Despite evidence for effective interventions, health services for children with chronic conditions--particularly mental health services--remain fragmented, signaling the need for increased attention to behavioral problems and their treatment among all health professionals caring for children.