Thymoma is a thymic epithelial neoplasm which induces T cell development. However, the frequency of mature CD4⁺ T cells in thymomas is lower than in normal thymi. Recently, CD4/CD8 lineage commitment has been elucidated in animal model. The zinc finger transcription factor Th-POK is a critical factor to CD4⁺ T cell development in CD4/CD8 lineage commitment, whereas CD8⁺ T cell development requires the transcription factor Runx3. These factors antagonize in CD4/CD8 lineage commitment. In this study, we examined Th-POK and Runx3 mRNA expression in the T cell subsets of human normal thymus and thymoma. A quantitative reverse transcriptase-polymerase chain reaction examination revealed that Th-POK expression in normal thymi was higher in the CD4⁺CD8⁻ subset than in the CD4⁺CD8⁺ and CD4⁻ CD8⁺ subsets. In thymomas, Th-POK expression in the CD4⁺CD8⁻ subset was significantly lower than that in normal thymi, and was significantly correlated with the proportion of CD3⁺ cells in the CD4⁺CD8⁻ subset. However, Th-POK expressions of the CD3⁺CD4⁺CD8⁺ and CD3⁺CD4⁺CD8⁻ subsets were not impaired in thymomas compared to normal thymi. These results suggest that thymoma neoplastic epithelial cells can induce Th-POK expression similarly to the normal thymic epithelial cells. In addition, there was no significant difference in Runx3 expression between normal thymi and thymomas. Therefore, CD4/CD8 lineage commitment dependent on Th-POK and Runx3 system seems to be working even in the neoplastic environment formed by human thymomas.