Depression is the major psychiatric complication of Parkinson's disease, and it occurs in approximately one-third of all cases. Diagnosis of depression is complicated by the overlap of depressive and parkinsonian symptoms. Its etiology is not fully understood but involves changes in dopamine and serotonin. Depression has significant clinical impact; depressed patients are more disabled (less independent). The depressive symptoms respond to treatment with antidepressants and electroconvulsive therapy.