Univariate and multivariate analyses of 3926 selected gastric cancer patients collected between 1981 and 1991 were made to confirm several clinicopathologic prognostic factors and to evaluate the effectiveness of postoperative immunochemotherapy with OK-432, 5-FU, and mitomycin C in stage III gastric cancer. Five-year survival rates for each category of prognostic variables were obtained; and in the univariate analysis, most of the factors (including age, depth of invasion, lymph node metastasis, location of primary tumor, histologic differentiation, and gross type except for sex) showed some significance. Multivariate analysis was conducted and verified significant prognostic factors. Depth of invasion and lymph node metastasis were found to be the most powerful factors (p < 0.001); gross type, location, and histologic differentiation were additional significant factors (p < 0.05). For postoperative treatment, immunochemosurgery was most effective in stage III patients (p < 0.05). In conclusion, we may predict a prognosis for gastric cancer more precisely on the basis of these independent prognostic factors, and immunochemosurgery may be a valuable means to treat advanced gastric cancer.