822 Background: In stage II colon cancer, retrieval of less than 12 lymph nodes (LNs) is a risk factor for recurrence. We previously reported that the great long-axis diameter of the largest retrieved lymph nodes (maximum LNs) is associated with a higher number of retrieved LNs and better outcomes (Int J Colorectal Dis 2015). We examined whether the long-axis diameter of maximum LNs is a prognostic factor in node-positive stage III colorectal cancer. Methods: The study group comprised 260 patients (pts) with stage III disease who underwent curative resection from 2005 through 2014. Pts who underwent laparoscopic or emergency surgery were excluded. The long-axis diameter was measured on H-E stained specimens and one non-metastatic LN with the greatest long-axis diameter was selected. The cut-off value (COV) for the long-axis diameter was determined by using receiver ROC curves with recurrence or overall mortality as events. Results: Tumors were located in right colon in 86 pts (33%), left colon in 104 pts (40%), and rectum in 70 pts (27%). In pts with rectal cancer, 49 (70%) received chemoradiotherapy (CRT). The mean long-axis diameter was 8.0 ± 3.5 mm. Factors related to the long-axis diameter were tumor location (right colon 9.5 ± 3.8 mm, left colon 7.7 ± 3.4 mm, rectum 6.6 ± 2.4 mm, p < 0.001), CRT (absent 8.4 ± 3.6 mm, present 6.3 ± 2.3 mm, p < 0.001), TNM stage (IIIA 6.6 ± 3.0 mm, IIIB 8.0 ± 3.5 mm, IIIC 8.8 ± 3.7 mm, p = 0.025), and the number of retrieved LNs (less than 12, 6.5 ± 2.9 mm; 12 or more, 9.0 ± 3.5 mm, p < 0.001). The COV for the long-axis diameter from the ROC curves was 8.0 mm. A long-axis diameter of ≥8.0 mm (106 pts 41%) were associated with significantly better outcomes than a long-axis diameter of < 8.0 mm (154 pts 59%) (5y RFS: < 8.0 mm 57%, ≥8.0 mm 70%, p = 0.012). On multivariate analysis, sex (female/male, HR 0.61, p = 0.019), venous invasion (absent/present, HR 0.58, p = 0.042), TNM stage (IIIB/IIIA, HR 3.19, p = 0.021; IIIC/IIIA, HR 3.47, p = 0.033), and the long-axis diameter (≥8.0 mm/ < 8.0 mm, HR 0.55, p = 0.005) were independent prognostic factors. Conclusions: In pts with stage III colorectal cancer, the long-axis diameter of maximum LNs was a prognostic factor. The size of maximum LNs might thus reflect the tumor immunity of the host.