T1 pancreatic cancers are small tumors. Despite their small size, their prognoses are extremely dismal compared to small cancers at other organs. However, because pancreatic cancers are infrequently found at an early stage, T1 pancreatic cancers are relatively rare. Therefore, little about T1 pancreatic cancer, including prognosis, clinical features, and the role of adjuvant treatment, has been studied. To overcome the limitation of low prevalence and provide better knowledge of T1 pancreatic cancer, the Korean Association of Hepato-Biliary-Pancreatic Surgery and the Japanese Society of Hepato-Biliary-Pancreatic Surgery have launched a collaborative investigation. Since the first proposal in June of 2018, more than three years have passed. During the period data from 1506 patients from 43 high-volume pancreatic centers across Korea and Japan have been successfully collected for analysis. Using large, yet granular data, we were able to shed light on clinicpathologic features of T1 pancreatic cancer and validate the T1 definitions in the eighth edition AJCC cancer staging system. In this study, we found that T1 pancreatic cancer displayed aggressive behavior despite its small size. The median survival duration of patients was 50 months, and the 5-year survival rate was 45.1%. R0 resection was unachievable in 10.0% of patients, the nodal metastasis rate was 40.0%, and recurrence occurred in 55.2%. Patients who received adjuvant treatment had a superior outcome compared to those who did not. This further highlights the importance of aggressive local and systemic treatment even in T1 pancreatic cancer. The definition of T1 pancreatic cancer in the eighth edition AJCC cancer staging system was explored for its feasibility. The current T1 subcategorization was not feasible for PDAC, whereas subdividing into two groups with 1.0 cm as the cut-off was feasible. Moreover, tumors with extrapancreatic extension (72.8%) had worse outcomes than those without extrapancreatic extension (median survival 107 vs 39 months, p<0.001). Extrapancreatic extension was an independent prognostic factor whereas the current T1 subcategorization was not. These results imply rooms for improvement of the AJCC staging system. Lastly, the results from the joint cohort were validated using the Surveillance, Epidemiology, and End Results (SEER) data. The fact that the results from the joint cohort could be reproduced from the SEER data demonstrated that the findings of this study are not findings unique only to Korea and Japan, but that may have global implication. The study is coming to an end with the completion of manuscript. The manuscript is currently in submission process. We sincerely hope that this study can have an impact on shaping the future pancreatic cancer staging system.