Despite enormous energies invested in authoring clinical practice guidelines, the quality of individual guidelines varies considerably. The Conference on Guideline Standardization (COGS) was convened in April 2002 to define a standard for guideline reporting that would promote guideline quality and facilitate implementation. Twenty-three people with expertise and experience in guideline development, dissemination, and implementation participated. A list of candidate guideline components was assembled from the Institute of Medicine Provisional Instrument for Assessing Clinical Guidelines, the National Guideline Clearinghouse, the Guideline Elements Model, and other published guideline models. In a 2-stage modified Delphi process, panelists first rated their agreement with the statement that "[Item name] is a necessary component of practice guidelines" on a 9-point scale. An individualized report was prepared for each panelist; the report summarized the panelist's rating for each item and the median and dispersion of rankings of all the panelists. In a second round, panelists separately rated necessity for validity and necessity for practical application. Items achieving a median rank of 7 or higher on either scale, with low disagreement index, were retained as necessary guideline components. Representatives of 22 organizations active in guideline development reviewed the proposed items and commented favorably. Closely related items were consolidated into 18 topics to create the COGS checklist. This checklist provides a framework to support more comprehensive documentation of practice guidelines. Most organizations that are active in guideline development found the component items to be comprehensive and to fit within their existing development methods.