Two cases of accessory spleen persisting after splenectomy for autoimmune thrombocytopenic purpura are reported. The diagnosis of accessory spleen must be considered whenever post-splenectomy blood smears fail to show pitted erythrocytes or Jolly's bodies in red cells. Isotopic methods, now completed by computerized tomography, will then confirm the persistence of splenic tissue. As illustrated by one of our patients, only large accessory spleens would produce a relapse of autoimmune thrombocytopenic purpura. In case of major thrombocytopenia a second splenectomy is worth attempting, although its benefits are inconstant and unpredictable.