We included in our study nine normal human thymuses and sixteen thymomas (1 type A, 2 type AB, 3 type B1, 3 type B2, 5 type B3 and 2 type C). The 25 patients were between 7 days and 75 years old, and were submitted to open surgery for correction of congenital heart defects or for mediastinal tumor mass. Biopsies were formalin fixed for 24 hours and then embedded in paraffin using routine procedure. Five micrometers step sections were mounted on silanized slides. Slides from each case were stained with haematoxylin and eosin method for morphological diagnosis. Additional sections were immunostained using monoclonal antibodies against high molecular weight cytokeratin clone 34beta E12. We found a very strong positive reaction for this type of cytokeratin in thymomas type B2, B3 and inconstant in type C comparative with normal thymus. Also, the number and distribution of positive epithelial cells in normal thymus versus thymomas were different. We found positive cells into capsular vessels of thymomas. This could be an invasion marker for apparent encapsulated thymomas. Strong positive reaction in almost all epithelial cells of thymomas types mentioned above was correlated in part with invasion. We concluded that expression of 34beta E12 cytokeratin is a useful marker for thymomas with high grade of malignacy, correlated with vascular and capsular invasion.