Clinical trials now have an established place in the evaluation of new cancer treatments. It is vital that there should be no conflict between the scientific aspects of clinical trials and the interests of patients with cancer. This conflict can be avoided if trial designs meet accepted standards, and if patients are able to make "informed" decisions abut participation. This process of obtaining informed consent is the subject of much debate. How the debate can be resolved by an individual clinician depends on his perception of a number of fundamental ethical principles. There is growing agreement, however, that regardless of the details of the process of obtaining informed consent, certain basic requirements must be met. The patient must be told that he is participating in a research procedure, that entry is optional, that withdrawal at any time is his right, and he must understand the nature of the trial and the likely effects of treatment, both good and bad.