Obsessive-compulsive disorder (OCD) is classified as an anxiety disorder and is characterized by obsessive thoughts or images causing distress and anxiety and attempts to relieve that distress through compulsive actions. The obsessions (sometimes termed "intrusive thoughts") are distressful to the individual, who recognizes that they are inappropriate but feels they are beyond his or her control. Inability to suppress these obsessions leads to activities, either mental (eg, counting) or physical (eg, hand washing), to quell the anxiety. The actions are not experienced as pleasurable. These obsessions and compulsions take up a considerable amount of time, over an hour daily, and can interfere with social functioning.

The most common obsessions include doubting, fear of contamination, and fear of losing control of impulses to commit aggressive or socially inappropriate acts. OCD involving contamination fears can be accompanied by hypochondriacal behavior, which may be a presenting feature. Evidence of dermatologic injury from excessive washing should raise the possibility of OCD. There is a high comorbidity between OCD and Tourette syndrome.

OCD symptoms may retrospectively be recognized in childhood, but is usually diagnosed in adolescence or early adulthood. The course is often one of waxing and waning symptoms, fluctuating with psychosocial stressors.

In adults, men and women are diagnosed with OCD in approximately equal proportions. Age of onset is either in childhood or young adulthood (bimodal); it is uncommon to see onset after age 30. In childhood-onset OCD, there is a male to female ratio of approximately 2-3:1. Boys may also have earlier onset of OCD.