"I don't believe I have ever heard a knowledgeable, well-trained psychiatrist make such a preposterous claim [that patients have a chemical imbalance], except perhaps to mock it...In truth, the 'chemical imbalance' notion was always a kind of urban legend—never a theory seriously propounded by well-informed psychiatrists." –Ronald W. Pies, M.D., Professor of Psychiatry, the State University of New York and Tufts University School of Medicine

From Psychology Today (click the link for more)