Problem behavior in children shows a high degree of cooccurrence, both within the domains of internalizing and externalizing behaviors and across domains. Maternal ratings of 207 same- and opposite-sex twin pairs on the Child Behavior Checklist (CBCL/4–18; Achenbach, 1991) were used to determine the etiology of these associations. In the current sample of 4- to 11-year-old children (mean age: 7.6 years) phenotypic correlations were .68 between Internalizing and Externalizing, between .41 and .66 within the internalizing scales, and between .58 and .71 within the externalizing scales. Correlations across domains on the scale level were of similar magnitude. Genetic and shared environmental correlations were generally high, with the exception of the somatic complaints scale which showed low genetic correlations with other scales. These findings of uniformly high shared etiology was seen within and across domains. For example, Attention Problems showed genetic correlations of .65 with Delinquent Behavior and .76 with Aggressive Behavior; the genetic correlations of Attention Problems with internalizing scales were .71 for Anxious/Depressed and .79 for Withdrawn; a low genetic correlation was only shown for the Somatic Complaints scale (rg = .16). Although the magnitude of shared environmental contributions was lower, the environmental correlations were close to unity, indicating that the same family environmental factors were involved in aspects of problem behavior when assessed across domains. Results for the Attention Problems scale were similar to other disruptive behavior, justifying its inclusion with the externalizing scales.