Objectives—To assess the relationship between the approval of trials by a research ethics committee (REC) and the fact that informed consent from participants (ICP) was obtained, with the quality of study design and methods. Design—Systematic review using a standardised checklist. Main measures—Methodological and ethical issues of all trials published between 1993 and 1995 in the New England Journal of Medicine, the Lancet, the Journal of the American Medical Association and the British Medical Journal were studied. In addition, clinical trials conducted in Spain and published by at least one Spanish author during the same period in any other journal were also included. Results—We studied the published articles of 767 trials and found the following indicators of lower methodological quality to be independent predictors for failure to disclose REC approval or ICP: absence of concealment of allocation, lack of justification for unblinded trials, not using a treatment for the patients in the control group, absent information on statistical methods, not including sample size estimation, not establishing the rules to stop the trial, and omitting the presentation of a baseline comparison of groups Conclusion—Trials of higher methodological and scientific quality were more likely to provide information about their ethical aspects.