In making treatment decisions, doctors and patients must take into account relevant randomised controlled trials (RCTs) and systematic reviews. Relevance depends on external validity (or generalisability)--ie, whether the results can be reasonably applied to a definable group of patients in a particular clinical setting in routine practice. There is concern among clinicians that external validity is often poor, particularly for some pharmaceutical industry trials, a perception that has led to underuse of treatments that are effective. Yet researchers, funding agencies, ethics committees, the pharmaceutical industry, medical journals, and governmental regulators alike all neglect external validity, leaving clinicians to make judgments. However, reporting of the determinants of external validity in trial publications and systematic reviews is usually inadequate. This review discusses those determinants, presents a checklist for clinicians, and makes recommendations for greater consideration of external validity in the design and reporting of RCTs.