Since Koprowski and coworkers discovered the CA 19-9 antigen 10 yr ago, it has become the most useful blood test in the diagnosis and management of patients with cancer of the pancreas. With an upper limit of normal of 37 U/ml, the assay's overall sensitivity is approximately 80% and its specificity is 90%. If higher cutoffs are used, the specificity rises so that, at levels greater than 1000 U/ml, the marker's specificity approaches 100%. Acute cholangitis and cirrhosis are two benign conditions that might raise this assay significantly. This tumor-associated marker is also helpful in predicting unresectability of pancreatic adenocarcinoma, as 96% of tumors that result in blood levels greater than 1000 U/ml have been found to be unresectable. After potentially curative surgery, the CA 19-9 can help prognosticate survival. Patients who normalize their CA 19-9 postoperatively live longer than those who do not. Furthermore, the assay, when used serially, predicts recurrence of disease prior to radiographic or clinical findings. The CA 19-9 is currently the "gold" standard marker for pancreatic cancer, against which other assays in this field will be judged.