Expression of the cell adhesion molecules including E-cadherin and its cytosolic binding proteins, alpha- and beta-catenins, has been widely studied in a variety of tumours, but not, to date, in thymic epithelial tumours. To observe the expression pattern of these adhesion molecules, immunohistochemical stains for E-cadherin (E-CD) and alpha- and beta-catenins were performed on 89 cases of thymoma which were classified as cortical (57 cases), mixed (18 cases), and medullary (14 cases), based on the classification of Marino and Müller-Hermelink. The majority of cortical thymomas showed diffuse and homogenous membrane immunoreactivity for these molecules (88 per cent for E-CD; 86 per cent for alpha-catenin; 91 per cent for beta-catenin) and the remaining cases showed heterogeneous immunoreactivity, whereas almost all mixed and medullary thymomas revealed decreased expression or were negative. In each histological subtype of thymoma, the expression did not correlate with invasion or with the presence of myasthenia gravis. These results indicate that the expression of E-CD and alpha- and beta-catenins is more closely associated with the histological subtypes of thymoma than with their biological behaviour.