The association of B cell chronic lymphocytic leukemia and pure red cell aplasia in a 42 year-old patient led to the discovery of a thymic enlargement. After six weeks treatment including steroids, cyclophosphamide and three courses of plasma exchange without improvement, surgical thymectomy was followed by a reticulocytosis and remission of the red cell aplasia. The tumor was a thymolipoma. Characteristics of this are thymic tumor, his connections with pure red cell aplasia and in vitro differentiation of erythroid progenitors are discussed.