Carbohydrate antigen (CA) 19-9 identified by a murine monoclonal antibody against a colorectal carcinoma antigen is thought to be a sialylated Lewis (Le)^a blood group antigen and occurs in high concentration in serum of patients with pancreatic carcinoma. This study was designed to identify the relationship between Lewis antigens and CA 19-9 in patients with pancreatic cancer. The following analyses were performed in 20 pancreatic cancer patients: Le^a and Le^b antigen phenotype in saliva (modified enzyme-linked immunosorbent assay) or on red cells (hemagglutination); CA 19-9 levels (radioimmunoassay) in serum; and CA 19-9 and Le^a and Le^b expression (immunoperoxidase assay) on tumor tissue. Le^a-b- patients based on salivary phenotype failed to express CA 19-9 in tumor tissue and had normal or low levels of CA 19-9 (<37 units/ml) in serum (P = 0.0011, versus Le^a+b- and Le^a-b+ patients). Eighty-eight % of Le^a+b- and Le^a-b+ patients had elevated serum CA 19-9 levels (>37 units/ml). All Le^a+b- and Le^a-b+ patients expressed both Le^a and Le^b antigens in tumor tissue. These results support the view that Le^a-b- pancreatic cancer patients cannot manufacture CA 19-9. Surprisingly, Le^a-positive patients express Le^b antigen in tumor tissue; in this subgroup, Le^b antigen may be a tumor-specific biomarker.