Objective: To review the literature on comparisons between different methods of obtaining informed consent for clinical trials. Design: Eight hundred and twelve articles were traced, in the process of conducting a systematic review of the ethics of clinical trials, by searching a number of sources: bibliographic databases (Medline, Psychlit and BIDS science and social science indices), hand searches, personal contacts, an original collection and a systematic follow-up of reference lists. Fourteen research reports were found which provided comparative data on different methods of obtaining informed consent. Eleven of these used a randomised design. Studies were classified according to three outcome measures (anxiety, consent rate and understanding). Results: The results of the various studies suggest that giving people more information and more time to reflect tends to be associated with a lower consent rate. There seems to be an optimal level of information about side-effects such that patients are not overburdened by detail, while grasping the most important risks. More information in general is associated with greater awareness of the research nature of the trial, voluntariness of participation, right to withdraw and (available) alternative treatments. This result does not, however, extend to explanations of the concept of randomisation on which the literature is contradictory -- sometimes more information is associated with increased understanding of the concept and sometimes it is not. Although divulging less information seems to be associated with less anxiety, there is evidence of an interaction with knowledge -- high levels of knowledge are significantly associated with less anxiety, irrespective of consent method. The more that patients know before they are invited to participate in a trial, the better equipped they are to cope with the informed consent procedure. Conclusion: There is some evidence to suggest that there is an optimal amount of information which enhances patient understanding and which might, in turn, reduce anxiety. However, the studies were not altogether conclusive. More work needs to be carried out, especially on public understanding of science and on how different ways of explaining scientific concepts affect that understanding.