Background: Heightened error and conflict monitoring are considered central mechanisms in obsessive-compulsive disorder (OCD) and are associated with anterior cingulate cortex (ACC) function. Pediatric obsessive-compulsive patients provide an opportunity to investigate the development of this area and its associations with psychopathology. Methods: Repeated measures were carried out using functional magnetic resonance imaging (fMRI) during the performance of an interference task, the arrow version of the Flanker paradigm, before and after cognitive-behavioral treatment of 25 medication-free pediatric obsessive-compulsive patients compared with age- and gender-matched healthy controls. Results: During error trials compared to correct trials, pediatric OCD patients and controls showed an interaction effect of Group x Time x Age in the ACC and insula. This effect was mainly driven by an increased activation in older OCD subjects, which was also present after treatment. During high-conflict trials compared with low-conflict trials, a Group x Time x Age interaction effect was found in bilateral insula. This effect was driven by an increase of BOLD (blood oxygen level dependent) signal in older OCD patients before but not after treatment. In addition, a Group x Time interaction effect in dorsomedial prefrontal cortex, premotor region and ACC was found. This effect was driven by an increase of BOLD signal in OCD subjects relative to controls over time. Conclusions: Compared to healthy controls, children and adolescents with OCD show increased activation of the ACC during error responses and in bilateral insular cortex during high-conflict tasks, which is age dependent and which is only partially affected by cognitive-behavioral therapy (CBT). Therefore, we suggest that ACC functioning is a vulnerability marker in pediatric OCD, whereas insular dysfunction may be state dependent.