Levels of a new carbohydrate antigen CA 19-9, which is a monosialoganglioside identified by a monoclonal antibody, were measured in 41 patients with pancreatic cancer and these serial changes were investigated. To provide a contrast, the serum CEA levels were compared with the serum CA 19-9 levels. As the result of this study, it was found that the serial changes of the serum levels of CA 19-9 were more correlative to the clinical course of these patients with pancreatic cancer than those of the serum CEA levels.