Serum expression of the cancer-associated antigens CA 19-9 and CA 50 and their relation to Lewis blood cell status were studied in 26 patients with pancreatic duct carcinoma and 26 patients with pancreatitis. The discriminating capacity between benign and malignant disease was high for both tumor markers. The correspondence between serum levels of CA 19-9 and CA 50 was close irrespective of the Lewis phenotype of the patient. All cancer patients with normal levels of CA 19-9 and CA 50 were of the phenotype Le(a-b-). Knowledge of the Lewis phenotype may therefore add vital information when tumor marker assays are used for diagnosis and monitoring of malignant pancreatic disease.