PURPOSE: In non-small cell lung cancer, higher thymidylate synthase (TS) levels have been reported in squamous cell carcinoma (SCC) compared with adenocarcinoma (ADC). Data on TS expression in large-cell carcinoma (LCC) are scanty. EXPERIMENTAL DESIGN: TS mRNA and protein levels were analyzed in 42 surgical cases of pulmonary LCC, including 8 large-cell neuroendocrine carcinomas, and were compared with controls represented by ADC (n = 41), SCC (n = 30), and small-cell lung carcinoma (SCLC; n = 33). TS levels were also correlated with the expression of Ki67 and E2F1. Moreover, the reliability of TS expression analysis was assessed in 22 matched cytologic and surgical specimens of non-small cell lung cancer. RESULTS: TS mRNA levels of LCC were comparable with those of control SCC, but significantly higher than those of ADC (P < 0.001) and lower than SCLC (P < 0.001). A correlation between TS mRNA and protein levels was observed in control ADC and SCC, but not in LCC. Large-cell neuroendocrine carcinomas had the highest TS expression, whereas in non-neuroendocrine LCCs, TS protein levels were significantly higher (P = 0.02) in LCC immunoreactive for p63 and desmocollin3 (markers of squamous differentiation) than those expressing TTF-1 (a marker of ADC). Both E2F1 and Ki67 levels were not correlated with TS in LCCs. Finally, a linear correlation in TS protein levels was observed between matched cytologic and surgical specimens. CONCLUSION: The pulmonary LCC immunoprofile may resemble that of SCCs or ADCs. This immunoprofile is associated with differential TS expression levels, which may support a more appropriate therapeutic strategy decision. (Clin Cancer Res 2009;15(24):7547-52).