To examine child factors and parental characteristics as predictors of discrepancies between parents' ratings of externalizing and internalizing behavior problems in a sample of preschool children with ADHD and behavior problems and in a nonclinical sample. We investigated correspondence and discrepancies between parents' ratings on the externalizing and internalizing behavior problems broadband scales of the Child Behavior Checklist version for preschool children (CBCL/1.5-5). Parents of 152 preschool children, with ADHD and behavior problems (n = 72) and nonclinical children (n = 80), aged between 28 and 72 months (M = 47.26, SD = 12.7), completed the CBCL/1.5-5. Candidate predictors of discrepancy included the child's age and sex, and parents' levels of parenting stress, depressive mood, attention-deficit and disruptive behavior. Hierarchical multiple regression analyses were conducted. Correspondence between parents, both for ratings on internalizing and externalizing behavior problems, was high (r = .63-.77). In the clinical sample, mothers rated the severity of externalizing behavior problems significantly higher than did fathers (p = <.001). Discrepancy between fathers and mothers on externalizing behavior problems was not predicted by child factors or interparental differences in psychopathology, but it was predicted by interparental differences in parenting stress (R (2) = .25, p <.001). This effect was significantly larger in the nonclinical sample (Delta R (2) = .06, p <.001). When parents disagree on the severity level of preschool children's externalizing behavior problems, the clinician should take into consideration that differences in parenting stress might be involved.