In the surgical treatment of advanced rectal cancer, wide lymphadenectomy has been replaced by autonomic nerve preservation because of serious problems such as postoperative urinary and sexual dysfunction. This may offer a considerable clue to as whether metastasis of lateral lymph node can be diagnosed by a new technique in order to provide more benefits to many patients with advanced rectal cancer. Moreover the sentinel lymph node (SLN) concept has lately attracted attention as a new technique. We studied SLN in 40 patients with advanced lower rectal cancer using 99mTc-Sn colloid as a tracer. SLN was successfully identified in 35 (87.5%) of 40 patients, and the mean number of SLNs identified per patient was 5.6 lymph nodes (range 1-14). In this study overall accuracy was 86%, and it was suggested that lateral lymphadenectomy was excluded in 91%, thanks to application of the SLN technique. Although the role this technique plays in the individualization of the therapy for rectal cancer will be large in the near future, many problems remain to be solved in the exact identification of SLN.