Establishing collaboration takes considerable time and effort. It can be difficult to trace the people responsible for eligible studies and they may be initially reluctant to participate in the meta-analysis. Often the first approach will be by letter, inviting collaboration, explaining the project, describing what participation will entail and how the meta-analysis will be managed and published. The letter is often from the project team and might be sent on behalf of the advisory group for the review. A protocol is generally supplied at this stage to provide further information, but data are not usually sought in the first correspondence. It may also be necessary to establish separate contact with the data centre or research organization who are (or have been) responsible for management of the study data, and to whom data queries will need to be sent. In encouraging the original investigators to take part in the IPD review, it is important to be as supportive and flexible as possible, to take the time required to build relationships and to keep all collaborators involved and informed of progress. Regular newsletters and e-mail updates can be useful ways of keeping the collaborative group up to date and involved, especially if the project will take place over a prolonged period.