Cancer chemoprevention envisions a reduction in the incidence of the disease through direct action using various chemical products. Chemoprevention trials involve healthy subjects, as a consequence careful ethical consideration must be given to such interventions. The classical contract between the patient and his doctor is altered in this situation and the numerous consequences of extrapolation of results to the general population must be considered. In particular a calculation of the risk-benefit ratio must take into account not only the physical but also the mental and social well being of people whose lives will be medicalised as a result of such intervention. The primacy of collective benefit over individual interest mandates that chemoprevention be based on scientific and well conducted trials. When results are extrapolated to the whole population an evaluation process has also to be permanently performed. The target population must be well informed and the participation based on voluntary consent.