Accurate identification of lymph nodes involved in metastases is vitally important for predicting survival, and it facilitates decision making with regard to adjuvant therapy. The study described here, which was undertaken to evaluate the role of sentinel lymph node mapping in refining the staging of colorectal cancer, was performed prospectively in 19 patients with colorectal cancer who underwent surgery from January to July 2005. Sentinel lymph node sampling was performed during each operation with isosulfan blue dye. Additional immunohistochemical staining was performed only if the sentinel nodes were negative for metastasis. In 18 of 19 patients, at least I sentinel node was identified. In 5 of 18 patients, sentinel nodes were positive for metastasis, and in 3 of 5, the sentinel node was the only node containing metastasis that was detected by immuncihistochemical staining. In 3 patients, metastases in nonsentinel lymph nodes were detected by hernatoxylin and eosin staining; these were determined to be false-negative results. Upstaging associated with sentinel lymph node mapping may reveal disease that might otherwise remain undetected by conventional methods. Patients who are upstaged may benefit from adjuvant therapies that have been shown to improve survival.