When a health plan denies payment for a procedure on grounds that it is not medically necessary or when it refuses a physician-ordered referral to a specialist, has it crossed the line from making an insurance judgment to practicing medicine? If the patient suffers harm as a result of the decision, is the plan liable for medical malpractice? Those are questions 29 states considered in 1998, and at least 35 states are grappling with this year as they seek to respond to physician and patient pressure to curb the power of the managed care industry. Traditionally, health insurers have been protected by state laws banning "the corporate practice of medicine," which means the patient's only recourse is to sue under a "vicarious liability" theory. Now, however, lawmakers are debating legislation to extend the scope of malpractice liability beyond individual practitioners to insurance carriers and plans themselves.