Arachnoid cyst masquerades as late onset obsessive-compulsive disorder.

Literature evidence suggests that onset of obsessive–compulsive disorder (OCD) at a later age is usually associated with brain lesions. However, none of previous reports suggest an association between arachnoid cyst and OCD. In this report, we present a case of OCD, starting at the age of 40 years, in which the obsessive symptoms were characteristically associated with fluctuating insight. Investigation revealed an arachnoid cyst, in the area of left fronto-parietal region, with broad base towards the falx.