Background: Pancreatic invasive ductal adenocarcinoma (PDAC) is the fourth leading cause of cancer mortality in Japan. The early diagnosis of pancreatic cancer, which will increase the number of patients with resectable tumors, is urgently needed. The purpose of the present study was to examine the earliest signs of pancreatic abnormalities on CT in order to facilitate the diagnosis and treatment of PDAC. Methods: Forty-one patients with pancreatic cancer and their 154 CTs were selected for the present study. We used the images that were acquired prior to the diagnosis and examined the pancreas in these images to observe serial changes in the morphology of the pancreas after selecting CT images in which PDAC was suspected. We also confirmed whether the main pancreatic duct was observed around that area of the pancreas. Four thousand two hundred seventy-seven patients without pancreato-biliary disease with 4630 CTs were selected for the control group. Results: Two pancreas shapes were detected: localized constriction of the pancreatic parenchyma referred to as the K-shaped sign, and localized fatty changes. Twenty-four (58.5%) of 41 patients showed the K-shaped sign. The main pancreatic duct without dilatation was noted around the K-shaped sign in 9 of the 24 patients. Eight of 41 patients (19.5%) showed localized fatty changes. Nine of 41 patients (21.9%) showed no abnormality. In the control group, only seven of 4277 patients (0.16%) showed the K-shaped sign. Conclusions: The K-shaped sign including localized fatty changes is the earliest CT sign that presents with pancreatic abnormalities. The K-shaped sign does not indicate PDAC itself but may predict its future development.