Serum SLX, CEA, SCC and NSE levels were serially measured in 266 patients with lung cancer and compared with those in 345 patients with benign respiratory disorders (BRD). The positive rate for CEA in lung cancer (44.4%) and the false-positive rate in BRD (15.3%) were the highest among the 4 markers. The positive rate for SLX in lung cancer (32.0%) was lower than that of CEA, while the false-positive rate for SLX in BRD (7.2%) was lower than that of CEA. The positive rate for SLX was highest in adenocarcinoma and correlated better with the clinical stages than did CEA. SCC and NSE were specifically elevated in squamous cell carcinoma and small cell carcinoma, respectively. Using these 4 markers, only 70.2% of patients were correctly diagnosed as having lung cancer or BRD. In monitoring treatment effect, only SLX showed a statistically significant correlation with regression and progression in adenocarcinoma, while NSE and SLX showed such a correlation in small cell carcinoma. Serum tumor markers seem to be less sensitive for the diagnosis of lung cancer than chest X-ray and sputum cytology, indicating that a search for more specific markers is still required. However, in monitoring treatment effect, SLX appeared to be suitable for adenocarcinoma, while NSE and SLX seemed to be useful in small cell carcinoma.