['Meta-analyses of randomized controlled trials (RCTs) provide the highest level of evidence regarding the effectiveness of interventions. Less is known about how they are used to inform the design and reporting of RCTs.', 'A sample of RCTs published in leading medical journals in 2007 was assessed to establish whether authors considered previous trials in the design of their trial. An approach to calculate the sample size required for a significant pooled effect in an updated meta-analysis was applied to a subsample of the RCTs to illustrate the ways in which the results of an existing meta-analysis can be incorporated into the planning and reporting of new RCTs.', 'Six of the 27 trials assessed (22%) reported the use of previous trial(s) for sample size calculations. Meta-analyses relating the results of the trial to previous research were cited in 37% (10 out of 27) of the report discussion sections. Previous evidence is formally incorporated into retrospective sample size calculations for three of the trials.'] Does empirical assessment suggest that existing evidence could be used more fully in designing randomized controlled trials?