Histologic material from 156 patients treated with surgery for cancer of the large bowel was studied with regard to tumor host interaction and with particular emphasis on the reactions of the regional lymph nodes. Prognostic evaluation of the tumor included the extent of the primary lesion according to Dukes' classification and the grade of differentiation of the carcinoma. Morphologic evidence of host resistance was judged by the presence and degree of lymphoplasmacytic infiltration of the tumor and the reactions of the regional lymph nodes, which were classified in four histologic patterns: lymphocyte predominance, germinal center predominance, lymphocyte depletion, and unstimulated. Of the factors evaluated, those that appeared to correlate best with five-year survival were the stage of the disease, the grade of differentiation of the carcinoma, the degree of lymphoplasmacytic infiltration of the tumor, the lymphocyte predominance pattern, and, to a lesser degree, the germinal center predominance pattern. None of the other morphologic features related to lymph nodes showed favorable prognostic significance. The relationship of morphology to host immune mechanisms is discussed.