Tumor markers including sialyl SSEA-1 antigen (SLX) were measured in 77 patients with lung cancer and 116 patients with benign respiratory diseases. Increased levels of serum SLX were observed in 57.6% (19/33) of adenocarcinoma of the lung, and an overall positive rate was 42.9% (33/77) in lung cancer cases. The false positive incidence of SLX in the sera from the patients with non-malignant diseases was as low as 8.6%. Among other tumor markers determined, serum CA 19-9, CEA had a relatively high positive rate, but the false positive incidence were not low. In cases of poor prognostic cases of with adenocarcinoma of the lung, SLX levels became levels higher than the previous ones, but not in cases of other cell types were not. Not only in the patients with lung cancer but also in those with benign respiratory diseases such as bronchiectasis and lung fibrosis, high levels of SLX seemed to suggest poor prognosis. These results indicate that SLX may contribute for monitoring and prognosis of the cases of lung cancer and other respiratory diseases.