p53 protein expression in 34 thymic epithelial tumors was examined immunohistochemically, and p53 gene mutation was detected in selected cases by DNA sequencing, using formalin-fixed and paraffin-embedded tissues. The tumors comprised 12 noninvasive thymomas, 9 invasive/metastatic thymomas, and 13 thymic carcinomas. All the tumors were immunoreactive for p53 protein. The p53-positive tumor cells in noninvasive thymoma were less than 10% (low expressor) in 7 cases and 10% to 50% (moderate expressor) in 5 cases. In invasive/metastatic thymoma, two were low expressors and seven were moderate expressors. In thymic carcinomas, there were nine moderate expressors and four high expressors (with > 50% positive cells). There was significant difference in p53 protein immunopositivity between thymic carcinoma and each of the noninvasive or invasive/metastatic thymomas. The DNA sequencing study confirmed the presence of p53 gene point mutation in all 10 cases examined, including three low expressors. These results suggest that p53 gene mutation is an early event in thymic tumorigenesis, and the p53 protein-positive cells increase with the progression of the tumor. Immunostaining reactivity of p53 may be a useful adjunct to differentiate thymic carcinoma from thymoma.