Despite years of research and hundreds of reports on tumor markers in oncology, the number of markers that have emerged as clinically useful is pitifully small. 1-3 Often, initially reported studies of a marker show great promise, but subsequent studies on the same or related markers yield inconsistent conclusions or stand in direct contradiction to the promising results. It is imperative that we attempt to understand the reasons that multiple studies of the same marker lead to differing conclusions. A variety of problems have been cited to explain these discrepancies, such as general methodologic differences, poor study design, assays that are not standardized or lack reproducibility, and inappropriate or misleading statistical analyses that are often based on sample sizes too small to draw meaningful conclusions. 4-11 For example, in retrospective studies, patient populations are often biased toward patients with available tumor specimens. Specimen availability may be related to tumor size and patient outcome, 12 and the quantity, quality, and preservation method of the specimen may affect feasibility of conducting certain assays. There can also be biases or large variability inherent in the assay results, depending on the particular assay methods used. 13-17 Statistical problems are commonplace. These problems include underpowered studies or overly optimistic reporting of effect sizes and significance levels due to multiple testing, subset analyses, and cut point optimization. 18 Unfortunately, many tumor marker studies have not been reported in a rigorous fashion, and published articles often lack sufficient information to allow adequate assessment of the quality of the study or the generalizability of study results. Such reporting deficiencies are increasingly being highlighted by systematic reviews of the published literature on particular markers or cancers. 19-25 The development of REMARK guidelines was a major recommendation of the NCI-EORTC First International Meeting on Cancer Diagnostics (From Discovery to Clinical Practice: Diagnostic Innovation, Implementation, and Evaluation) that was convened in Nyborg, Denmark, in July 2000. The purpose of the meeting was to discuss issues, accomplishments, and barriers in the field of cancer diagnostics. Poor study design and analysis, assay variability, and inadequate reporting of studies were identified as some of the major barriers to progress in this field. One of the working groups formed at the Nyborg meeting was charged with addressing statistical issues of poor design and analysis, and reporting of tumor marker prognostic studies. The guidelines that we present in this article are the product of that committee. The Program for the Assessment of Clinical Cancer Tests (PACCT) Strategy Group of the US NCI has also strongly endorsed this effort (http://www. cancerdiagnosis. nci. nih. gov/assessment/). The guidelines that we present in this article build on earlier suggestions21, 26-29 and on educational publications. 30-33 The guidelines recommend elements and formats for presentation with the objectives of facilitating evaluation of the appropriateness and quality of study design, methods, analyses, and improving the ability to compare results across studies. As for the successful CONSORT initiative for randomized clinical trials, 34 and the STARD statement for studies of diagnostic test accuracy, 35 these