Histologic parameters which are thought to reflect either cell-mediated (T cell) or humoral (B cell) immune responses in lymph nodes have been studied in regional lymph nodes draining carcinoma of the sigmoid colon. Patients whose lymph nodes show morphological evidence of cell-mediated immunity, manifested either by an increased number of paracortical immunoblasts or sinus histiocytosis, survive significantly longer than those whose lymph nodes show no such changes. Patients whose lymph nodes show simultaneous paracortical activity and sinus histiocytosis have the best survival of all. Of this latter group, 11/13 (83 percent) are living without signs of recurrent tumor 5 or more years after surgery. Histologic parameters which suggest an antibody-mediated immune response (germinal center activity) were not an important prognostic indicator. The occurrence of favorable lymph node histology does not appear to significantly correlate with the modified Duke's classification. Rather, the favorable changes allow selection of a large proportion of those patients within the various Duke's categories who are destined to become long-term survivors.