In histology and cell block sections, antibodies to thyroid transcription factor-1 (TTF-1) and p63 have been demonstrated to be useful markers for distinguishing between small-cell lung carcinoma and poorly differentiated pulmonary squamous cell carcinoma. In this study, we assessed the utility of TTF-1 and p63, as an antibody panel, for differentiating between these two neoplasms in previously Papanicolaou (Pap)-stained cytologic smears and cytospin slides. Twenty-six lung carcinomas (13 small-cell lung carcinomas, 13 poorly differentiated pulmonary squamous cell carcinomas) were evaluated. One or two previously 95% ethanol-fixed, Pap-stained smears or cytospin slides were selected from each case. The cytologic material from these slides was transferred to positively charged slides. Unstained recuts were obtained from the corresponding histologic specimens or cell blocks. Immunohistochemical staining for TTF-1 and p63 was performed on the paired samples from each tumor. All (13/13) small-cell lung carcinomas were negative for p63 and 92% (12/13) were positive for TTF-1. Conversely, all (13/13) poorly differentiated pulmonary squamous cell carcinomas expressed p63 and did not express TTF-1. Immunoreactivity for p63 was also noted in bronchial reserve cells and metaplastic squamous cells. The immunostaining results obtained from the cytology slides were concordant with those of the histology or cell block sections in all cases. The results of this study show that TTF-1 and p63 immunostaining can be successfully applied to previously Pap-stained cytologic material, as an antibody panel, to facilitate pathologic differentiation between small-cell lung carcinomas and poorly differentiated pulmonary squamous cell carcinomas. p63 immunostaining, however, must be interpreted in conjunction with cytomorphology to distinguish between poorly differentiated pulmonary squamous cell carcinomas and benign cellular constituents of the lung.