Sialyl Lewis(a) (CA19-9) and sialyl Lewis(x) antigens (SLX) may play a role in tumor metastasis by serving as functional ligands in the cell adhesion system. The authors examined preoperative serum levels of CA19-9 and SLX in 218 patients who underwent resection for gastric cancer to determine their prognostic value. The patients were divided into two groups, termed the low and high antigen groups, based on a value selected as a diagnostic cutoff. Correlation between the antigen serum levels, various established clinicopathologic factors, and prognosis were studied by univariate and multivariate analysis. The disease-specific interval for high CA19-9 and SLX groups was significantly shorter than that of their respective low groups (p = 0.0024 and p < 0.0001, respectively). Patients with stage III/IV tumors who had high serum SLX levels had shorter disease-specific intervals than those with low serum levels (p = 0.0017). A Cox's regression analysis revealed a high serum SLX level as an independent factor for worse outcome. In addition, logistic regression analysis revealed that a high serum SLX level was an independent predictor for liver metastasis. In conclusion, an elevated preoperative serum SLX level was a predictor for poor outcome after resection for gastric cancer, whereas CA19-9 was not.