To examine neurobiological changes underlying obsessive-compulsive symptoms (OCS) we examined intrapair differences in behavior and fMRI brain activation in monozygotic twins discordant for OCS, using a Tower of London planning paradigm. Despite only mild evidence for impairment at the behavioral level, twins with OCS showed significantly decreased brain activation during planning in dorsolateral prefrontal cortex, thalamus pulvinar, and inferior parietal cortex. These findings are consistent with the hypothesis of disturbed cortico-striato-thalamo-cortical (CSTC) circuitry underlying OCS. In contrast to previous studies in patients with obsessive-compulsive disorder (OCD) we did not find robust evidence for reduced responsiveness in striatal brain regions. Together, these findings suggest that neurobiological mechanisms underlying OCS of environmental origin partly overlap with neurobiological changes in patients with OCD, where the disorder is likely caused by a combination of genetic and environmental influences. A difference between genetical and environmental etiologies may relate to the amount of reduced striatal responsiveness. © 2008 Elsevier B.V. All rights reserved.