An authoritative monograph by Emile Weil,1 entitled "La ponction de la rate," marked the beginning of definitive findings in normal and pathological hematological conditions. This was followed by Moeschlin's splendid monograph,2 which brought the subject up to date and served to stimulate others in furthering the study of spleen aspiration. Influenced by these researches, it was our intention to make a diagnostic study of spleen aspiration in hematological material that presented itself to us in hospital, clinic, and office. The scope of this contribution is limited to our own observations, since the literature has been adequately reviewed by Block and Jacobson.3 The time-honored examination of the peripheral blood, coupled with an adequate clinical investigation, sufficed to make possible the diagnosis of most hematological entities. In this manner, disorders of the red blood cells, white blood cells, and platelets were readily apparent. Anemias, erythrocytosis, agranulocytosis, leukocytosis, lymphocytosis, thrombocytopenia,