This study was conducted to investigate the clinical utility of CEA, CA 19-9, and CA-50 in the diagnosis, monitoring, and prognosis of 62 gastric carcinoma patients having either adjuvant or palliative chemotherapy. Patients were divided in two groups: group A included patients treated on an adjuvant basis following a curative resection of gastric cancer, and group B included patients with residual disease post surgery or patients with inoperable tumor or generalized disease. Serum marker levels were measured in a prospective study just before the initiation of chemotherapy and before each course during chemotherapy. In group A, CEA was positive in 2/25 (8%) patients, CA 19-9 in 1/25 (4%), and CA-50 in 1/25 (4%). In group B the sensitivity of CEA was 48.6% (18/37 patients), of CA 19-9 64.9% (27/37 patients), and of CA-50 70.3% (26/37) patients. There was a significant correlation between the CA 19-9 and CA-50 levels in both groups. No correlation was found between the sensitivity or the absolute initial marker levels and the tumor's differentiation or extent of disease. In group A the only patient with initially elevated CA 19-9 and CA-50 values relapsed early while he was on adjuvant chemotherapy. It was also found that the rising final CA 19-9 and CA-50 values at the end of chemotherapy were correlated with an increased incidence of relapse, but not with the disease-free interval. In group B the initially low marker levels showed a trend to predict a favorable outcome of treatment. There was no statistically significant correlation between the marker titers before each course and response to chemotherapy. It is concluded that the comeasurement of CA 19-9 and CA-50, and to some degree of CEA, is justifiable for gastric cancer. The estimation of CA 19-9 and CA-50 may be useful for early detection of recurrence after curative surgery and adjuvant chemotherapy. In advanced or recurrent gastric cancer, the estimation of either CA 19-9 or CA-50 and CEA serum values may help in checking the prognosis, determining the efficacy of palliative treatment modalities, and recognizing recurrences.