The value of sialic acid in serum, lung tissues and surface of blood cells of 43 patients with lung cancer were measured by means of spectrophotometry. Results obtained were compared with those of 12 normal individuals.Sialic acid in serum of normal individuals was 450±90μg/ml while that of lung cancer cases was 680±145μg/ml, which showed a statistically significant increase in the latter (p<0.01).The value of serum sialic acid in lung cancer patients (in adenocarcinoma and squamous cell carcinoma) showed a statistically significant increase compared with that of the control (p<0.01).In squamous cell carcinoma, serum sialic acid increased with the clinical stage.In 6 lung cancer cases, the value of sialic acid in tumor was higher than in normal tissues.The value of sialic acid on the erythrocyte cell membrane of lung cancer patients was higher than in normal cases.The value of sialic acid in autologous rosette-forming cells was closely related to rosette formation ability.From the above sialic acid would appear useful as a tumor marker for lung cancer.