Rheumatoid arthritis is associated with numerous abnormalities measurable in a laboratory. These are most commonly used to monitor disease but can also provide prognostic information; less frequently they are used to support a clinical diagnosis. In general, quantitative markers are used for monitoring early disease. By contrast, qualitative markers can provide prognostic information of particular relevance to early therapy. The markers with the best features for predication of radiological erosions are the MHC conserved epitope and rheumatoid factor. In established disease most of the quantitative measures indicate poor prognosis. Therefore, the choice of the best marker depends not only on the purpose for which it is desired, but also on the stage of disease.