This paper reports on implications of bipolar disorder (BD) co-morbidity in 120 children and adolescents with obsessive-compulsive disorder (OCD) (84 males, 36 females, age 13.7 +/- 2.8 years), diagnosed using a clinical interview according to the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) criteria, and naturalistically followed-up for 12 +/- 6 months. The aim of this naturalistic, retrospective study was to explore the effect of BD co-morbidity, disentangling it from other co-occurring variables, namely the co-morbidity with disruptive behavior disorders. Forty three patients (35.8%) had a bipolar co-morbidity. Compared with OCD patients without BD, they had an earlier onset of OCD, a greater severity and functional impairment, more frequent hoarding obsessions and compulsions, and a poorer response to treatments. They had a higher co-morbidity with attention-deficit/hyperactivity disorder (ADHD) and oppositional-defiant disorder (ODD), and a lower co-morbidity with generalized anxiety disorder (GAD). Finally, they received more mood stabilizers, and 30.2% of them did not receive serotonin-selective reuptake inhibitors (SSRIs) because of pharmacological (hypo)mania. When all the OCD responders and nonresponders were compared, nonresponders (n = 42, 35%) were more severe at baseline and at end of the follow-up, had more frequently hoarding obsessions and compulsions, and had more frequent BD, ODD, and conduct disorder (CD) and less GAD and panic disorder. In the final regression model, hoarding obsessions and compulsions, co-morbidity with ODD, and CD were negative predictors of treatment outcome. This study suggests that even though bipolar co-morbidity is frequent and affects phenomenology and co-morbidity in pediatric OCD, its effect on treatment response seems prevalently accounted for by co-morbidity with disruptive behavior disorders. The significance of the hoarding subtype deserves further research on larger samples of pediatric patients.