Serum levels and the incidence of elevated levels of several tumor markers were measured in 30 patients with chronic renal failure (CRF) of different degrees, as well as in 36 hemodialyzed (HD) patients without clinical evidence of neoplasia. The tumor markers evaluated were carcinoembryonic antigen (CEA), CA 125, CA 15.3, CA 19.9, CA 50, alpha-fetoprotein, neuron-specific enolase (NSE), squamous cell carcinoma antigen (SCC), prostatic acid phosphatase and prostatic-specific antigen. Serum levels of CEA were above the cutoff limit in 33% of patients with CRF and 47% of HD patients, CA 50 was higher than normal values in 37 and 44% of patients, respectively. SCC was elevated in 43 and 72% of patients, respectively. Serum levels of CA 125 were elevated in 18% of patients with CRF and NSE in 36% of HD patients. In CRF several tumor markers (CEA, SCC, CA 50 and NSE) show a high false positive rate and may be unreliable for monitoring malignancies in uremic patients, while the other markers evaluated appear to maintain their specificity in this situation.