To evaluate the usefulness of CYFRA 21-1 and SCC Ag in the diagnosis of squamous cell carcinoma (SQC) of the lung, we tested sera from 124 patients with lung cancers (squamous cell ca 72, adenoca 22, large cell ca 4, small cell ca 18 and undetermined 8) and 78 patients with inflammatory lung diseases (bronchitis 24, bronchiectasis 29, tuberculosis 19 and others 6) using immunoradiometric assay kit for cytokeratin fragment 19 (CYFRA 21-1) and radioimmunoassay kit for SCC Ag. The serum CYFRA 21-1 and SCC Ag were significantly higher in lung cancer patients compared with control subjects. However, the significant difference was restricted only to SQC. In patients with SQC, CYFRA 21-1 and SCC Ag showed significantly higher levels according to the advanced anatomic stages (stage I-IIIa vs. stage IIIb, IV, p < 0.05). There was a good correlation between CYFRA 21-1 and SCC Ag (r = 0.41, p < 0.001). Receiver operating characteristic (ROC) curves were generated from results of both tumor markers and areas under the curves (AUC) were calculated. AUC of CYFRA 21-1 (0.93) were significantly larger than that of SCC Ag (0.77) for the diagnosis of SQC (p < 0.05). Therefore, we conclude that CYFRA 21-1 is superior to SCC Ag in the diagnosis of squamous cell carcinoma of the lung.