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.