The results of detailed studies of 3 patients with essential thrombocythaemia (ET) are presented. Qualitative tests of platelet function were abnormal, and autologous platelet survivals were diminished. Megakaryocyte ploidy distribution analysis showed an increased proportion of cells with high ploidy, and buoyant density distribution analysis showed increased proportions of less dense platelets. Ultrastructural studies of the platelets showed proliferation of the dense tubular system with deficiency of the surface-connected canalicular system. Megakaryocyte electron microscopy showed abnormal distribution of demarcation membrane complex and granules.The precise functional abnormality of the platelets in ET has not yet been defined, but a release defect of variable severity with corresponding morphological abnormalities of the dense tubular and surface-connected canalicular systems, seems the most likely explanation.