Prognostic significance of host-immune response, as gathered by the degree of tumour lymphocytic infiltration (TLI), and its relationship to other prognostic variables were investigated in 361 colorectal cancer patients admitted to our Institution for curative resection from January 1960 to December 1978. The presence of a local immune reactivity was significantly related to a less advanced stage of disease and a better differentiated tumour. A poorer prognosis was detected in patients with minor or no lymphocytic infiltration. TLI was the single most important prognostic parameter, according to the Cox model and to logistic regression analysis. These findings suggest that also TLI should be considered in the current Staging System of colorectal cancer.