Background: The inclusion of grey literature (i.e. literature that has not been formally published) in systematic reviews may help to overcome some of the problems of publication bias, which can arise due to the selective availability of data. There is now some evidence in support of this, suggesting that the exclusion of grey literature from meta-analyses can lead to an exaggeration of the effect of treatment.

Objective: This study aims to review systematically research studies, which have investigated the impact of grey literature in meta-analyses of randomized trials. A study will be considered eligible for this review if it compares the effect of the inclusion and exclusion of grey literature on the results of meta-analyses of randomized trials.

Methods: Studies will be identified by searching the Cochrane Methodology Register, MEDLINE, EMBASE, Science Citation Index and by handsearching journals and conference proceedings. The main outcome measure will be an estimate of the impact of trials from the grey literature on the pooled effect estimates of the meta-analyses. Information will also be collected on the area of health care, the number of meta-analyses, the number of trials, the number of trial participants, the year of publication of the trials, the language and country of publication of the trials, the number and type of grey and published literature, and methodological quality.

Results and Conclusions: As of February 2002, two studies have been identified which assess the impact of including grey literature in meta-analyses of randomized trials. The results of these studies and any other studies identified in the interim will be reviewed.