CA72-4: a new tumour marker for gastric cancer. To date, tumour markers for gastric cancer have proved unreliable. In this study the value of a new serum marker, CA72-4, was compared with the serum activities of carcinoembryonic antigen (CEA) and CA19-9 in a consecutive series of patients with gastric cancer. The results show that the CA72-4 assay is significantly better at separating stage I and II disease from normal controls (P less than 0.01) than CEA (n.s.) or CA19-9 (n.s.). CA72-4 also gave better differentiation between patients with positive and negative nodes (P less than 0.01) and between those who were serosa positive and negative (P less than 0.01). CEA differentiated between patients with positive and negative nodes (P less than 0.05) but CA19-9 could not. CA19-9 and CEA could not discriminate between patients who were serosa positive and negative. In this study, at a specificity of 95 per cent, the sensitivities of CEA, CA19-9 and CA72-4 were 0.25, 0.41 and 0.94 respectively. These preliminary findings indicate that CA72-4 is a reliable tumour marker of disease stage and activity in gastric cancer. Further longitudinal studies are required for full evaluation of its clinical utility. 