Physicians who participate in clinical trials are mandated or encouraged to adhere to fundamental conditions than enhance the scientific validity of medical research. In some situations compliance with the study protocol may threaten the delivery of optimal care and raise ethical dilemmas among the staff. Moral conflicts may occur with respect to recruitment, control groups, randomization, blind assignments, patient withdrawals, and the use of supplementary therapies that could weaken statistical power. In designing and managing clinical trials, steps can be taken to reduce the likelihood of conflict between scientific inquiry and medical practice. Special means of protecting patients in the study can be introduced. Recognizing the existence of ethical dilemmas is a necessary prelude to the quest for objective solutions.