For a meta-analysis to give definitive information, it should meet at least the minimum standards that would be expected of a well-designed, adequately powered, and carefully conducted randomised controlled trial. These minimum standards include both qualitative characteristics--a prospective protocol, comparable definitions of key outcomes, quality control of data, and inclusion of all patients from all trials in the final analysis--and quantitative standards--an assessment of whether the total sample is large enough to provide reliable results and the use of appropriate statistical monitoring guidelines to indicate when the results of the accumulating data of a meta-analysis are conclusive. We believe that rigorous meta-analyses undertaken according to these principles will lead to more reliable evidence about the efficacy and safety of interventions than either retrospective meta-analysis or individual trials.