Serum concentrations of monoclonal antibody-defined tumor markers CA 19-9 and CA 50 were measured in 64 uncomplicated hemodialysis patients (Group 1) and in 8 hemodialysis patients with cancer (Group 2) in comparison with corresponding CEA determinations. From our results in Group 1 patients it appears that both CA 19-9 and CA 50 maintained an excellent specificity in these patients. As to the sensitivity of these tests, our findings in Group 2 patients are comparable with those reported in patients with cancer and normal renal function. It is concluded that, unlike CEA, these new tumor markers maintain their clinical value in chronic renal failure.