Anxiety disorders frequently occur in association with PD and may be important causes of morbidity. Actual prevalence rates are uncertain, but estimates suggest that up to 40% of patients with PD experience substantial anxiety. This percentage is greater than expected, particularly for an elderly population. In addition, the age at onset of anxiety in PD (and particularly panic disorder) is later than would be expected from current information regarding the natural course of anxiety disorders. Virtually all of the types of anxiety disorders have been described in PD, but panic disorder, GAD, and social phobia appear to be the ones most commonly encountered. Although most patients with motor fluctuations experience greater anxiety during the "off" phase, this is not a universal phenomenon. Anxiety frequently develops before the motor features do, suggesting that anxiety may not represent psychological and social difficulties in adapting to the illness but rather may be linked to specific neurobiologic processes that occur in PD. Most evidence points to disturbances in central noradrenergic systems, but other neurotransmitters (e.g., serotonin, dopamine) may be involved as well. Studies suggest that right hemispheric disturbances may be particularly important for the genesis of anxiety, especially panic and OCD. Whether antiparkinsonian medications themselves contribute to anxiety needs clarification. Anxiety and depression frequently coexist in PD. It remains to be determined whether anxiety in patients with PD reflects one of the following pathologies: (a) an underlying depressive mood disorder, (b) a particular subtype of depression (atypical depression, anxious or agitated depression), or (c) an independent psychiatric disturbance. The relationship between anxiety and dementia in PD is not clear, but current evidence suggests that cognitive dysfunction is not related to the presence of anxiety symptoms in this disorder. The optimal pharmacologic treatment for anxiety in patients with PD has not been established, nor has the effect of PD surgery on anxiety symptoms.