BACKGROUND For cancer patients, prognosis is strongly influenced by the completeness of tumor removal at the time of cancer‐directed surgery or disease remission after nonsurgical treatment with curative intent. These parameters define the relative success of definitive treatment and can be codified by an additional subclassification within the TNM system, the residual tumor (R) classification. Despite the importance of residual tumor status in designing clinical management after treatment, misinterpretation and inconsistent application of the R classification frequently occur that diminish or abrogate its clinical utility. METHODS An analysis of the relevant literature regarding the use and prognostic importance of the R classification was undertaken. RESULTS In the current study, the prognostic importance of the R classification for different kinds of tumors is discussed. Problems that arise in using the R classification are described. Special issues regarding the use of the R classification are addressed. CONCLUSIONS The R classification is a strong indicator of prognosis and facilitates the comparison of treatment results if applied in a consistent manner. Uniform use and interpretation of this classification is essential for the standardization of posttreatment data collection. Cancer 2002;94:2511–9. © 2002 American Cancer Society. DOI 10.1002/cncr.10492 This communication addresses the importance of the TNM residual tumor (R) classification, clarifies its definition, and offers guidelines for its appropriate application.