Pleomorphic carcinoma (PC) of lung is a poorly differentiated epithelial neoplasm predominantly composed of pleomorphic giant and/or spindle tumor cells. The WHO classification of lung cancer recognizes spindle cell carcinoma and giant cell carcinoma as separate neoplasms related to squamous cell carcinoma (SqC) and large cell carcinomas, respectively. Further, the presence of foci of SqC or adenocarcinoma (AdC) in, respectively, 10% and 45% of PC produces additional uncertainty as to the distinctive nature of this tumor type. In this study, the authors tested the hypothesis that PC is an entity separate from SqC or AdC by evaluating the mutational spectrum seen in these tumor types. This is performed by documenting and comparing mutation type and rate of K-ras-2 and p53 genes in PC, SqC, and AdC. Comparative DNA sequence and immunohistochemical analysis were performed on 22 PC, 42 SqC, and 97 AdC. Archival formalin-fixed, paraffin-embedded tissues formed the basis of the study. Immunohistochemical staining with p53 antibody (DO-7) revealed statistically significant differences in the intensity and frequency of staining of PC (weak, 86% of cases) versus SqC (strong, 52% of cases) and AdC (strong, 27% of cases) (P < .001). Topographic genotyping with subsequent polymerase chain reaction (PCR) and sequence analysis of K-ras-2 showed mutations in significantly fewer cases of PC (9%, 2 of 22 cases) than in AdC (36%, 35 of 97 cases) or SqC (0%, 0 of 42 cases) (P < .001). Pleomorphic carcinoma also showed significantly fewer p53 point mutations (14%, 3 of 22 cases) than did AdC (27%, 26 of 97 cases) of SqC (43%, 18 of 42 cases) (P < .01). Finally, the p53 mutations in PC were more common in exon 7, whereas those in SqC and AdC were more frequent in exon 8. These findings reveal significant differences in the pattern and frequency of genetic mutations between PC and pulmonary SqC and AdC and are in keeping with the separate histopathologic classification of these tumors.