|BACKGROUND: After JCOG0212 trial, more patients may undergo lateral lymph node dissection (LLND) outside Japan. LLN involvement (LLN(+) ) should be properly implemented to the TNM. AIM: to clarify the significance of LLND with respect to tumor staging. METHODS: We analyzed the cohort data from 22 institutions of the Japanese Study Group for Postoperative Follow-up of Colorectal Cancer. A total of 1,197 patients who had undergone potentially curative resection with LLND for rectal cancer between 1997 and 2006 were eligible. The mTNM-A staging system is with slight modification, if main LNs are positive (Main (+) ) or LLN(+), the N-category was defined as N2b regardless of the number of LNs involved (NLNI). In the mTNM-B staging system, N2c is further added and defined as NLNI >6 together with Main(+) or LLN(+). The TNM, mTNM-A, and mTNM-B staging systems were compared based on the AIC, and C-index with respect to relapse-free survival (RFS). RESULTS: The 5-year RFS for patients in TNM N2a and N2b were 55.7 and 36.9%, while mTNM-B N2a, N2b, and N2c were 58.0, 44.8, and 15.3%, respectively. The AIC for TNM, mTNM-A, and mTNM-B staging systems for Stage III patients are 3361, 3344, and 3335, while C-index are 0.633, 0.647, 0.658, respectively (p<0.05). CONCLUSIONS: Patients who underwent LLND were significantly better staged by the use of modified N category.