We aimed to determine semiquantitative evaluation criteria for Crohn-like lymphoid reaction (CLR). We reviewed 1,032 patients with colorectal cancer and evaluated CLR by counting all peritumoral lymphoid aggregates (LAs) and by measuring the maximum diameter of the largest LA. The maximum diameter of the largest LA, rather than the number, had a significant impact on survival. Active CLR determined by the 1-mm rule was significantly associated with MLH1/MSH2 immunohistochemical staining deficiency. The group with LAs 1 mm or larger had lower recurrence (P = .0008) and a higher survival rate (P < .0001) than that without LAs 1 mm or larger. These results were similarly observed in another cohort of 500 patients with colorectal cancer. The k values for CLR evaluation among 8 observers were 0.67 for the 1-mm rule and 0.50 for Graham's criteria. The size of the largest LA best reflects the specific characteristics of CLR, and the 1-mm rule is expected to improve assessment reproducibility.