Serum alpha(1----3)-L-fucosyltransferase activity was measured in 58 patients with lung cancer, 27 benign diseases, and in 100 healthy controls. The levels of enzyme activity were significantly higher in the sera of patients with cancer when compared to those in benign diseases and healthy controls. The elevation of the enzyme activity correlated with the clinical stages and to the size of the primary tumors. Follow-up studies with various stages showed that the enzyme activity was useful in tracking the clinical course of disease after surgery. To evaluate the usefulness of this enzyme as a diagnostic marker, carcinoembryonic antigen (CEA) and sialyl Lewis X-i antigen levels were also measured. The results indicate that alpha(1----3)-L-fucosyltransferase could be a more specific tumor marker than such tumor-associated antigens in lung cancer.