Cancer studies involving human subjects present ethical conflicts which require a delicate balance to be struck between respect for fundamental human rights and the need to obtain sufficient data to satisfy study goals. Scientific and ethical conflicts at all stages of application of the scientific method are discussed. Legislative safeguards and ethical guidelines continue to evolve. Both must form an integral part of considerations regarding the feasibility of conducting cancer studies involving human subjects. Researchers, administrators, and legislators must be educated in and remain sensitive to two competing forces in epidemiologic research: the need to know versus respect for the individual's right to privacy, safety, dignity, and autonomy. An enhanced role for Institutional Review Boards to include an interactive monitoring function with ongoing studies is proposed.