Background: Dysfunction of frontal-striatal, particularly orbitofrontal-striatal, circuitry has been implicated in the pathophysiology of obsessive-compulsive disorder (OCD), characterized by obsessions, ritualistic behavior, anxiety, and specific cognitive impairments. In addition, neuropsychological studies in OCD have reported impairments in visuospatial tasks and executive functions, such as planning. Objective: To determine whether dorsal prefrontalstriatal dysfunction mediates planning impairment in patients with OCD. Design: A parametric self-paced pseudorandomized event-related functional magnetic resonance imaging version of the Tower of London task was used in 22 medication-free patients with OCD and 22 healthy control subjects. This paradigm, allowing flexible responding and post hoc classification of correct responses, was developed to compare groups likely to differ in performance. Results: Behavioral results showed significant planning impairments in OCD patients compared with control subjects. During planning, decreased frontalstriatal responsiveness was found in OCD patients, mainly in dorsolateral prefrontal cortex and caudate nucleus. In addition, OCD patients showed increased, presumably compensatory, involvement of brain areas known to play a role in performance monitoring and short-term memory processing, such as anterior cingulate, ventrolateral prefrontal, and parahippocampal cortices. Conclusions: These findings support the hypothesis that decreased dorsal prefrontal-striatal responsiveness is associated with impaired planning capacity in OCD patients. Because the described frontal-striatal dysfunction in OCD is independent of state anxiety and disease symptom severity, we conclude that executive impairment is a core feature in OCD.