A distinctive Crohn's disease-like reaction (CLR) consisting of discrete lymphoid aggregates, some with germinal centers, and surrounding stellate fibrosis is commonly found around colorectal adenocarcinomas in the absence of clinical or pathologic evidence of prior Crohn's disease. Most lymphoid aggregates are located one or more millimeters beyond the advancing tumor fronts, mainly in muscularis propria and pericolic adipose tissue. The intensity of this reaction was graded as absent, mild, or intense for 100 consecutive colorectal carcinomas. These tumors also were evaluated for invasion beyond muscularis propria, lymphoid infiltrates at the tumor margin, and nodal metastases. The degree of CLR was correlated with survival. Results indicated that CLR is more likely to occur in transmurally invasive carcinomas than in those confined to the colonic wall, especially in the right side. An intense CLR is associated with an intense lymphoid infiltrate at the tumor edge, a lower incidence of nodal metastases, and a statistically significant increase in 10-yr survival. These results suggest that the Crohn's-like reaction around colorectal carcinomas may be a favorable host response, similar to lymphocytic infiltrates at the advancing tumor edge. Analysis of more cases should clarify whether this CLR is independent of other variables.