Acquired amegakaryocytic thrombocytopenic purpura (AATP) is a haematological disorder characterized by severe thrombocytopenia due to an immunologically induced absence of megakaryocytes in an otherwise normal-appearing bone marrow. A 57-year-old male with a 6-month history of rectal and cutaneous bleeding is reported. Platelet count was 10 X 10(9)/l, while other haematological values were within the normal range, except for the presence of hypogammaglobulinaemia with decreased IgA and IgG. Both platelet median volume and half-life span were normal, and antiplatelet IgG determinations were negative. Bone marrow aspiration and biopsy showed no megakaryocytes, with a normal appearance of erythroblastic and granulopoietic series. An in vitro culture for megakaryocytic progenitor cells did not show any growth of megakaryocyte colonies. No inhibitory effect on the growth of normal marrow megakaryocytic colonies was observed when serum and lymphocytes of the patient were added. Following 4 weeks of prednisone therapy, the platelet count rose to 127 X 10(9)/l and the bone marrow aspirate showed some megakaryocytes. The possible pathogenetic mechanisms of this entity are discussed.