A number of serum tumor markers are clinically relevant for primary lung carcinomas. None of them, however, is applied to screening of lung cancers because of their unsatisfactory sensitivity and specificity. Among them, measurements of CEA, CYFRA 21-1, NSE, and pro GRP frequently give subsidiary information as to differential diagnosis, monitoring of treatment, and early detection of recurrence. Current trend of the tumor markers for lung cancer includes development of new markers such as p53 tumor suppressor gene product and I-CTP for detecting bone metastasis. Attempts to detect micrometastases by means of RT-PCR of these marker genes are also discussed.