Pretreatment serum levels of the tumor markers carcinoembryonic antigen (CEA), CA 50, and CA 19-9 in 95 patients with squamous cell carcinoma of the esophagus and 32 age-matched controls were compared. Thirty-nine percent of the cancer patients showed elevated (greater than or equal to 5 micrograms/l) serum CEA levels, 41% had elevated (greater than or equal to 17 U/ml) CA 50 levels, and 13% showed elevated (greater than or equal to 37 U/ml) CA 19-9 levels. The tumor markers showed a considerable degree of complementarity, and combined tumor marker analysis increased the sensitivity to 59%. Raised CEA levels were found significantly more frequently in intrathoracically localized tumors than in cervical cancers. Patients surviving less than 6 months showed a higher rate of elevated CEA assays than those who survived 6 to 18 months. No certain correlation was established between tumor marker elevation and tumor stage or tumor differentiation.