BACKGROUND: The Core Outcome Measures in Effectiveness Trials (COMET) initiative aims to facilitate the development and application of 'core outcome sets' (COS). A COS is an agreed minimum set of outcomes that should be measured and reported in all clinical trials of a specific disease or trial population. The overall aim of the Core Outcome Measurement Instrument Selection (COMIS) project is to develop a guideline on how to select outcome measurement instruments for outcomes included in a COS. As part of this project, we describe our current efforts to achieve a consensus on the methods for selecting outcome measurement instruments for outcomes to be included in a COS. METHODS/DESIGN: A Delphi study is being performed by a panel of international experts representing diverse stakeholders with the intention that this will result in a guideline for outcome measurement instrument selection. Informed by a literature review, a Delphi questionnaire was developed to identify potentially relevant tasks on instrument selection. The Delphi study takes place in a series of rounds. In the first round, panelists were asked to rate the importance of different tasks in the selection of outcome measurement instruments. They were encouraged to justify their choices and to add other relevant tasks. Consensus was reached if at least 70% of the panelists considered a task 'highly recommended' or 'desirable' and if no opposing arguments were provided. These tasks will be included in the guideline. Tasks that at least 50% of the panelists considered 'not relevant' will be excluded from the guideline. Tasks that were indeterminate will be taken to the second round. All responses of the first round are currently being aggregated and will be fed back to panelists in the second round. A third round will only be performed if the results of the second round require it. DISCUSSION: Since the Delphi method allows a large group of international experts to participate, we consider it to be the preferred consensus-based method for our study. Based upon this consultation process, a guideline will be developed on instrument selection for outcomes to be included in a COS.