33 patients with chronic myelo-proliferative disease were investigated for megakaryocytes in cubital venous blood. 6 Patients had chronic myeloid leukaemia, 11 polycythaemia vera and 16 myelofibrosis. In all patients, a significantly higher number of megakaryocytes than in normal adult humans was found. In patients with chronic myeloid leukaemia and myelofibrosis, an increased number of intact megakaryocytes was found, indicating thrombopoiesis in the peripheral part of the upper extremity. 2 patients with myelofibrosis showed a % of intact megakaryocytes greater than 25, as found in blood from the inferior vena cava in normal adults. In patients with chronic myeloid leukaemia, some of the megakaryocytes were smaller than normal, whereas in myelofibrosis, some of the megakaryocytes were larger than normal, with increased lobulation of nuclei. Active thrombopoiesis in the peripheral part of the extremities can be of great importance, when splenectomy is considered in treatment of myelofibrosis.