Lawyers do not testify; witnesses do. No matter how compelling the opening and closing statements of the trial lawyer, every juror is instructed that he or she is bound to reach conclusions that are based not on what the lawyers say, but on the evidence. In a professional malpractice lawsuit, the most important evidence that the jury hears is the testimony of the expert witness. When a newspaper reports a jury verdict against a defendant doctor, it is fair to assume that at some point during the trial another doctor sat in the witness chair, identified himself as an expert, described the applicable standard of medical care, and testified under oath that the defendant deviated from that standard. Put more simply, before a doctor can be found liable for malpractice, some other licensed doctor generally must testify against him. The Tort Policy Working Group appointed by the United States Attorney General in October 1985 recently observed that important causation findings made by juries in medical malpractice cases have increasingly come to be “based on fringe scientific and medical opinions well outside the mainstream of accepted scientific or medical beliefs.” Given the important role of the expert doctor in any jury trial, this observation is alarming and calls to mind a number of issues. When should a doctor agree to testify as an expert in a malpractice case? What restraint, if any, should be exercised by the medical expert in giving testimony? Given the virtual sea of malpractice cases that have been filed and the chilling effect they have had on the practice of medicine and dentistry, should doctors ever agree to testify as experts against one another? These are questions that raise both the legal and ethical considerations explored in this article.