To evaluate the correlation between serum levels of sialyl Lewis X-i antigen and distant metastasis and survival in patients with non-small cell lung cancer (NSCLC), we measured the serum levels of the tumor marker in 371 patients with untreated NSCLC. The sialyl Lewis X-i antigen level was measured using a RIA kit. In patients with adenocarcinoma or other NSCLC subtypes, there was a correlation between serum sialyl Lewis X-i antigen and stage of the disease (P = 0.0001 and P = 0.0015, respectively). Levels of the marker varied significantly depending on the number of metastatic organs in adenocarcinoma (P = 0.0089) and in other NSCLC subtypes (P = 0.002). Univariate analysis showed that survival of NSCLC patients with high (more than 100 units/ml) sialyl Lewis X-i antigen levels was significantly poorer than that of patients with low antigen levels (P = 0.0001). Multivariate analysis using Cox's proportional hazard model showed that high sialyl Lewis X-i antigen levels correlated significantly with poor survival (P = 0.004). Our data suggest that a high serum level of sialyl Lewis X-i antigen seems to be an indicator of the presence of metastasis and might indicate the need for a careful investigation of all putative metastatic sites. The serum levels of sialyl Lewis X-i antigen may reflect the extension of metastasis and would be helpful in considering treatment options.