Lymphocytic infiltration was assessed semiquantitatively in 447 specimens of rectal cancer. Corrected five year survivals for pronounced, moderate, and little lymphocytic infiltration were 92%, 65% and 36%, respectively. Grading was shown to be reproducible in an intraobserver study (observed agreement 81%, expected agreement 34%, kappa coefficient 0.72). In the multivariate survival analysis of grade and stage related variables lymphocytic infiltration was the only grade related variable to be accepted within the prognostic model. This model also included the number of lymph node metastases and the extent of tumour spread. It is suggested that the conservation of the normal interactive traffic between epithelium and mucosa associated lymphoid tissue (MALT) signals a low grade growth. This is supported by the association between lymphocytic infiltration and the expression of secretory component by malignant epithelium.