BACKGROUND The role of serum tumor markers in non-small cell lung cancer (NSCLC) remains undefined. New proposed markers have seldom been rigorously compared with existing standards. The authors prospectively compared the performance of three new monoclonal antibodies (MoAb) (5E8, 5C7, and 1F10) with the established serum markers carcinoembryonic antigen (CEA) and squamous cell carcinoma antigen (SCC). METHODS The cohort consisted of 45 consecutive out-patients with newly diagnosed NSCLC: Control subjects were 38 outpatients with non-neoplastic chronic pulmonary diseases. Blood from each patient and control subject was assayed for all five tumor markers. An enzyme-linked immunosorbent assay (ELISA) was used to determine 5E8, 5C7, and 1F10 reactivity. Commercially available kits were used to measure SCC by radioimmunoassay and CEA by ELISA: Individual and combinations of tumor markers were compared in terms of sensitivity, specificity, and accuracy for NSCLC diagnosis. RESULTS 5E8 plus 5C7 plus 1F10 significantly surpassed SCC plus CEA in terms of sensitivity (P < 0.05) and proved the most accurate marker combination. Among single markers, 5E8 was most specific, 5C7 most sensitive, and 5C7 and 1F10 each most accurate, but differences from CEA alone were not significant. Subgroup analysis by histologic type and stage demonstrated similar findings, and marker combinations yielded little additional diagnostic benefit. CONCLUSIONS 5E8, 5C7, and 1F10 performed marginally better than did CEA and SCC in patients with newly diagnosed NSCLC: Many limitations apply in defining a clinical niche for these tumor markers in NSCLC, although 5E8, 5C7, and 1F10 previously have demonstrated a modest prognostic value. An adjunctive role in a few specific clinical contexts remains possible.