The overall incidence of malignancy in renal transplant recipients is 100-fold higher compared with age matched controls. Routine clinical evaluation therefore often includes the determination of serum tumor markers AFP, CA19-9, CEA, CA125, CA15-3, PSA, and calcitonin. We evaluated the specificity and the sensitivity of these markers in 575 renal allograft recipients. Specificity varied between 0.69 (CA 125) and 0.96 (PSA) in 532 patients without cancer. Cyclosporine therapy and excretory allograft function did not affect marker concentration; impaired liver function was associated with significantly elevated AFP, CA19-9, CA125, and CA15-3 levels. In 43 patients with malignancies the sensitivity of the markers ranged between 0.2 (CEA) and 1 (CA 125, CA 15-3). We therefore conclude that routine screening of the transplant population with serum tumor markers is not useful because of the low sensitivity and specificity of these tests.