Clinical presentation and treatment outcome for children with comorbid externalizing and internalizing symptoms Rhea M. Chase * , Sheila M. Eyberg University of Florida, FL, United States Received 10 April 2006; received in revised form 5 March 2007; accepted 12 March 2007 Abstract This study examined the effects of comorbid separation anxiety disorder (SAD) on the expression of externalizing symptoms in children presenting with oppositional defiant disorder (ODD) as well as the treatment effects on anxiety and internalizing symptoms. Participants were 64 children with ODD seen in parent–child interaction therapy (PCIT), including 15 children with comorbid SAD. Children with ODD + SAD did not differ from children with ODD only in disruptive behavior severity at pre- treatment assessment, and children with ODD + SAD showed significant decreases in SAD symptoms at post-treatment. Additionally, children with clinical levels of internalizing behavior demonstrated significant reductions in these symptoms, along with reduction of externalizing symptoms targeted in treatment. We discuss the possibility that treatments focusing on parent–child interactions and certain parenting skills may generalize across specific child symptom constellations. # 2007 Elsevier Ltd. All rights reserved. Keywords: Parent–child interaction therapy; Separation anxiety disorder; Oppositional defiant disorder; Comorbidity; Internalizing behavior; Externalizing behavior High rates of multiply diagnosed children have led to increased attention to the diagnosis of comorbid psychological disorders. Comorbidity introduces sev- eral important issues, particularly when considering effective treatment options for young children. Limited research exists on treatment course and outcomes for children with comorbid psychological disorders, and the research that does exist largely focuses on homotypic comorbidity. For example, researchers have examined treatment implications for comorbid anxiety and depression (Kendall, Kortlander, Chansky, & Brady, 1992); and comorbid attention-deficit hyper- activity disorder and conduct disorder (Abikoff & Klein, 1992). However, a significant number of children experience symptoms of both anxiety disorders and disruptive behavior disorders (Jackson, Frick, & Dravage-Bush, 2000; Verduin & Kendall, 2003; Wright, 2001). Current literature largely fails to address the effects of co-existing externalizing and internalizing symptoms on symptom expression and treatment outcome. Wright (2001) found that the presence of separation anxiety disorder (SAD) actually exacerbated the rate of disruptive behavior in a sample of boys. This suggests that comorbid disorders are not entirely separate entities. Rather, co-existing disorders may influence each other in both presentation and course. One study using parent–child interaction therapy (PCIT) to treat disruptive behaviors in preschoolers found significant improvements in internalizing as well Anxiety Disorders 22 (2008) 273–282 * Corresponding author. Tel.: +1 352 273 5238; fax: +1 352 273 6156. E-mail address: rchase@phhp.ufl.edu (R.M. Chase). 0887-6185/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.janxdis.2007.03.006