Ethical conflicts between moral principles and methodologic standards sometimes occur in epidemiologic research. When dilemmas are discerned, they may be analyzed using the ethical principles of beneficence, nonmaleficence, justice, and respect for the autonomy of persons. We argue that, in addition to scientific validity, the welfare and rights of research subjects should be taken into account in making decisions regarding all aspects of the design and conduct of epidemiologic studies, and that the commitment of epidemiologists to the advancement of scientific knowledge should not outweigh or override all other considerations.