DEPRESSION is the most common of psychiatric symptoms and occurs in many psychiatric illnesses. Major depressive disorder can be distinguished by sleep and appetite disturbances and other "hypothalamic" signs and symptoms.1-4 Major depression can be subdivided into unipolar and bipolar depressive illness on the basis of a history of mania in bipolar illness (Fig 1), and into primary and secondary depression.1-4 Secondary depression and minor depressive illness occur in medical illness and in nonaffective psychiatric illness such as obsessive-compulsive neurosis, personality disorders, schizophrenia, drug and alcohol dependence, and sexual dysfunction, respectively. These categories (Fig 1) have been helpful in improving diagnostic reliability between psychiatrists and reducing heterogeneity in diagnostic groups. They improve understanding of pathophysiology and facilitate prediction of treatment response and prognosis. Biologic tests have been developed that can support a clinical impression or diagnosis. A number of important biochemical subgroups of unipolar major depression can be