For the past ten columns, we have been exploring questions of differential diagnosis and the ways to apply the Diagnostic and Statistical Manual of Mental Disorders, third edition, to the complexity of the clinical situation. We are now embarking on an even more uncharted and certainly more controversial area: differential therapeutics. In this column, to appear in alternate issues, we will learn how our guest experts would select treatment modalities for various clinical situations.We have the good fortune to lead off this new series with Toksoz B. Karasu, M.D., professor of psychiatry at Albert Einstein College of Medicine in the Bronx and director of psychiatry at the Bronx Municipal Medical Center. Dr. Karasu is the chairman of the American Psychiatric Association's commission on psychiatric therapies and has been deeply involved in studying bow best to determine which treatments are most effective for which psychiatric disorders in which patients and circumstances.