Prospective randomized controlled clinical trials are a powerful tool for unbiased comparative evaluation of alternative treatment policies. Such trials must include relatively large numbers of patients, usually beyond the resources of any one clinical centre. Collaboration between centres is usually necessary and inevitably causes problems of communication at all stages of trial design and execution. Trial design demands agreement on the prior data base, and the writing of a trial protocol. The protocol must include precise depiction of trial objectivsa, eligibility criteria, randomization procedures, treatment policies, the end points by which effectiveness will be measured, and the records to be kept. It will also contain sections describing the scientific background and rationale of the trial, and statistical and ethical considerations. Once the trial is in progress, information must flow from the participating clinical institutions to a coordinating data centre; this will usually require the use of specially designed data collection forms. Most trials will require separate forms for initial registration, pre‐therapy baseline information, sequential evaluation (flow sheet), and summaries when the patient reaches defined end‐points. Additional forms may document specific treatment details, evaluation by a reviewer, and patient self‐assessment scores or autopsy details. Attention to these aspects of study design will avoid later problems of misinterpretation, and will facilitate accurate and timely acquisition of data. Information flow from the coordinating centre to trial participants is also important, in order to maintain interest and patient ac