Background: In order to minimize publication bias, authors of systematic reviews often spend considerable time trying to obtain unpublished data. These include data from studies conducted but not published (unpublished data) as either an abstract or full text paper as well as missing data (data available to researchers but not reported) in published abstracts or full text publications. The effectiveness of different methods used to obtain unpublished or missing data has not been systematically evaluated. Objectives: To assess systematically effects of different methods for obtaining unpublished studies (data) and missing data from studies to be included in systematic reviews. Methods: Primary studies comparing different methods of obtaining unpublished studies (data) or missing data were identified by using a sensitive search strategy to search the Cochrane Methodology Register, MEDLINE and EMBASE (1980 to February 2009). Results: Of 3836 identified abstracts and titles, 19 potentially eligible publications, referring to 18 studies, were selected. Following independent eligibility assessment, four studies met the inclusion criteria, six are awaiting assessment and eight were excluded. Preliminary results show that having a well-known signatory had no significant effect on the likelihood of authors responding to a request for unpublished information. Another study found that leg-work ahead of time to clarify and request the specific unpublished study information required from the drug industry proved to be more fruitful than sending of a non-specific request. One study found that correspondence with study authors by e-mail resulted in the greatest response rate with the fewest attempts and shortest time to respond whereas another study using either e-mail or letter to provide a semi-personalised information retrieval sheet, did not provide much useful data. Conclusions: Eligibility assessment and data extraction are ongoing and will be presented at the Colloquium. Results from this review will assist review authors in improving the efficiency in conducting their reviews.