A case of a myasthenic patient with the association of unifocal histiocytosis X (eosinophilic granuloma) and lymphoid follicular hyperplasia of the thymus is presented. The combined diagnosis was made on histological grounds and supported by immunohistochemistry. Two aspects of this case are of interest: (1) it is the first reported case of the association of histiocytosis X and lymphoid follicular hyperplasia of the thymus in a myasthenic patient; and (2) the pathogenesis of this peculiar association does not seem to be fortuitous but rather might be related to the general derangement of the immune system present in myasthenia gravis.