The effects of exogenous opioids are completely opposite to anxiety symptomatology and suggest the existence of a relationship between opioid systems and Anxiety Disorders. The involvement of opioid systems in Obsessive-Compulsive Disorder (OCD) is suggested by its analogy with other Anxiety Disorders and by the role of such system in disorders of the obsessive-compulsive spectrum (Tourette's Disorder, Substance-Related Disorders, Eating Disorders, Impulse-Control Disorders); some experimental evidence in patients affected by OCD and the effect of therapy with opioid agonists and antagonists further support this hypothesis. The involvement of endogenous opioids in Posttraumatic Stress Disorder is widely supported. Opioid involvement in Panic Disorder is suggested by pharmacological evidence, such as naltrexone-induced Panic Attacks; moreover, the opioid system may decrease activity of the locus coeruleus. Finally, patients affected by Generalized Anxiety Disorder have higher beta-endorphin plasma levels than controls, before facing stressful situations. Therefore, these data suggest a possible opioid involvement in core psychopathological aspects of OCD, related to pleasure and reward systems.