Pre-operative computed tomography (CT) of the anterior mediastinum was performed in eight patients undergoing surgery for cardiac conditions. At operation the anterior mediastinum was generously biopsied. Histology of the material allowed evaluation of the proportion of normal residual thymic tissue. Wide variation in CT appearances and in the degree of thymic involution was found, but there was good correlation between CT findings and thymic histology. This normal variation is of importance when considering the CT diagnosis of anterior mediastinal soft tissue density tumours.