OBJECTIVE: Behavior problems are common in children with functional constipation. This study assessed the prevalence of overall, internalizing, and externalizing behavior problems in children with functional constipation and explored which clinical characteristics of constipation are associated with these behavior problems. METHODS: Children who had functional constipation, were aged 4 to 18 years, and were referred to the gastrointestinal outpatient clinic at the Emma Children's Hospital were eligible for enrollment. This study made use of baseline data of 133 children who participated in a randomized, controlled trial that evaluated the clinical effectiveness of behavioral therapy compared with conventional treatment. Prevalence of behavior problems was assessed by the Child Behavior Checklist. Univariate and multivariate logistic regression models were used to test the association between clinical characteristics and behavior problems. RESULTS: The prevalence rate of overall, internalizing, and externalizing behavior problems was considerable: respectively 36.8%, 36.1%, and 27.1% compared with 9% in the Dutch norm population. A long duration of treatment was found to have the strongest association with overall and externalizing behavior problems in children with constipation. Children with constipation and nighttime urinary incontinence have an increased risk for having overall behavior problems. Fecal incontinence and the production of large stools seemed to be exclusively related to externalizing behavior problems. CONCLUSIONS: Behavior problems are common in children who have constipation and are referred to gastrointestinal outpatient clinics, suggesting that a behavioral screening should be incorporated into the diagnostic workup of children with constipation. Copyright © 2010 by the American Academy of Pediatrics.