This paper reports three cases of lupus erythematosus (LE) associated with thymomas, all of which were discovered after the onset of LE. The diagnosis of LE was established by clinical and laboratory data. Histologically, the thymomas were of the three main types: epithelial, lympho-epithelial, and predominantly fusiform. In two cases, thymectomy did not modify the course of LE; in one vase, the absence of cytoplasmic secretory vesicles in the epithelial cells and the non-labelling of these cells by an antiserum thymic factor monoclonal antibody represented direct evidence of a functional thymus deficiency. Antikeratin antibodies were used to distinguish thymomas from lymphomas; epithelial thymomas exhibited the same keratin specificities as normal thymus, although the labelling pattern (net-like) was different. Labelling of epidermis with the same antiserum confirmed the theory of an epidermis-thymus relationship.