BACKGROUND: Traditionally, large tumor size is associated with poor survival in colon cancer, but its impact remains limited and controversial. Recently, the impact of small tumor size on prognosis has gained attention. This study aimed to investigate whether small tumor size can be an additional parameter for T3N1 colon cancer prognosis. METHODS: We retrospectively analyzed 162 consecutive patients with pT3N1 colon cancer between 2010 and 2021. The optimal cutoff value of tumor size was calculated through receiver operating characteristic curve analysis. We evaluated survival through the Kaplan-Meier method, and the risk of prognosis through multivariate Cox models. RESULTS: The optimal cutoff value of tumor size was 45 mm. The recurrence rate was significantly higher in tumor size <
45 mm than in ≥ 45 mm (25.7% vs. 12.0%, p = 0.037). Tumor size <
45 mm also had lower overall survival (OS), recurrence-free survival (RFS), and cancer-specific survival (CSS) than its counterpart (p = 0.03 for all). In multivariate analyses, age ≥ 70 years, undifferentiated histological type, and tumor size <
45 mm were independent prognostic factors for OS (p = 0.025, p = 0.002, p <
0.04, respectively), whereas tumor size <
45 mm was the only independent prognostic factor for RFS (p = 0.043). Meanwhile, the independent prognostic factors for CSS were undifferentiated histological type and tumor size <
45 mm (p = 0.008 for both). CONCLUSIONS: Small tumor size is associated with poor prognosis in pT3N1 colon cancer cases. Thus, small tumors potentially have biologically aggressive features.