Extent of the tumor in carcinoma of the pancreas seems an important factor in stage classification. The purpose of this report is to present the results of studies on the relationship between the size of the tumor (T) and prognosis, and the correlation between the size and the other factors influencing prognosis such as invasion to the pancreatic capsule, invasion to the retroperitoneal tissue, metastasis to the regional lymph node, and so on. T was divided into four groups: T1, T2, T3, T4. Tumor diameter of 2 cm or less was designated as T1, 2.1-4 cm as T2, 4.1-6 cm as T3, and 6.1 cm or more as T4. Case reports of 163 patients who underwent resection of carcinoma of the pancreatic head were submitted by 10 major institutions and retrospectively analyzed. There was significant differences in 1-year cumulative survival rates between T1 and T3, T1 and T4, and T1 and T2 + T3 + T4, but no significant difference in 3- or 5-year survival rates among the T groups. The incidence of no capsular invasion, no retroperitoneal invasion, and no lymph node metastasis had a tendency to decrease along with increase in tumor size and the incidence of definite capsular invasion, and lymph node metastases increase with increase in the size. Attempts to diagnose tumors of less than 2 cm should be made to ameliorate the poor surgical results. However, it should be stressed that even in the T1 group regional lymph node metastases were seen in about half of the cases.