Intraoperative sentinel lymph node (SLN) detection has been reported for colon cancer, but no study has focused on rectal cancer. Only an ex vivo technique can be performed easily in this location. We evaluated SLN detection using blue dye injection in patients with rectal adenocarcinoma. This prospective study included 31 patients. Preoperative radiotherapy (45 Gy) was done in 15 cases. After proctectomy the surgical specimen was examined in the operating room. Submucosal peritumoral injections were done. One to three SLNs were retrieved. The SLNs were sectioned at three levels and examined histologically and then, if negative by hematoxylin-eosin (H&E) staining and immunohistochemistry (IHC). There were 7 abdominoperineal resections, 12 colorectal anastomoses, 11 coloanal anastomoses, and 1 Hartmann procedure. The median number of lymph nodes harvested was 21 (7-38). A SLN was identified in 30 cases (feasibility 97%). The mean number of SLNs was 2 (0-3). A micrometastasis was discovered in 3 of 23 pNO cases when H&E was used on multisection levels, thus changing the stage to pN1. Each time the only positive lymph node was the SLN. IHC evaluation did not change the result, as only isolated tumor cells were discovered in one case. Only four of seven N+ patients had a positive SLN, resulting in a false-negative rate of 43%. Ex vivo detection of SLNs is possible for rectal cancer and is a simple technique. Classic analysis using H&E remains the gold standard. However, SLNs detection can change the tumor stage by upstaging nearly 15% of the tumors from T2-3N0 to T2-3 N+.